Click below to display this full wiki and read if you have general questions about using GUCs -- they may be answered here!
NOTE: Both GUC and RUC rules are listed below because the way GUCs work on domestic routes is now different than how RUCs work

New Fare Table for GUC/RUC Effective 2/1/2022

*****GUC*****

Flight has First Class
MC/C+ = Purchased
First = Last Seat J (Old Policy = OY)

Flight has DPS only
MC/C+ = Purchased
DPS = Last Seat (GUC=P / RUC=G) (Old Policy = RY)

Flight has D1 with no DPS
MC/C+ = Purchased
D1 = Z (Old Policy = OY)

Flight has D1 with DPS
MC/C+ = Purchased
DPS = Last Seat (GUC=P / RUC=G) (Old Policy = RY)
D1 = OY @ T-24h15m

Flight has D1 with DPS
DPS = Purchased
D1 = Z (Old Policy = OY)

Note: If GUC applied to domestic flight, it has better inventory than RUC (better chance for D1 transcon or to Hawaii)

*****RUC*****

Flight has First Class
MC/C+ = Purchased
First = Z (Old Policy = OY)

Flight has D1 with no DPS
MC/C+ = Purchased
D1 = OY (no change in old policy = OY)

Delta is using public inventory buckets J Z P G (and non-public bucket OY in some cases) to determine GUC/RUC availability. However once they have confirmed you the system converts the segment in your reservation to a different bucket. Here is the class mapping:
GUC Usage
MC or C+ purchased / Upgrade to DPS / P class needed / Books into RV (Clears at T-24.15 into D1 as OX if OY available)
MC or C+ purchased / Upgrade to D1 (no DPS) / Z class needed / Books into OK
DPS purchased / Upgrade to D1 / Z class needed / Books into OK
MC or C+ purchased / Upgrade to FC (no D1/DPS) / J class needed / Books into OV

RUC Usage
MC or C+ purchased / Upgrade to DPS / G class needed / Books into RX (Clears at T-24.15 into D1 as OX if OY available) ****THIS AIRCRAFT CONFIG MAKES NO SENSE DPS IS NOT SOLD ON RUC ROUTES, MAYBE THIS IS FOR FUTURE PLANS?****
MC or C+ purchased / Upgrade to D1 (no DPS) / OY class needed / Books into OX
DPS purchased / Upgrade to D1 / OY class needed / Books into OX ****THIS AIRCRAFT CONFIG MAKES NO SENSE DPS IS NOT SOLD ON RUC ROUTES, MAYBE THIS IS FOR FUTURE PLANS?****
MC or C+ purchased / Upgrade to FC (no D1/DPS) / Z class needed / Books into OL

New Delta.com Upgrade Availability Indicators that appeared 2/1/22 and what they mean

Dom D1 Tag meaning:
Global Upgrade Certificate Available = GUC in Z (OY = 0 for RUC)
Upgrade Certificate Available = GUC in Z /or/ RUC in OY both available

Dom F Tag meaning:
Global Upgrade Certificate Available = GUC in Last Seat J (Z = 0 for RUC)
Upgrade Certificate Available = GUC in last seat J /or/ RUC in Z both available

Example D1 Flights from JFK - LAX (flight numbers are fictitious)

DL 01
Dom D1 flight
Actual Availability: J5 C4 D4 Z3
Hidden Availability: OY2
Website indicator: Upgrade Certificate Available
Actual decode: (2 RUC (OY) open, 3 GUC (Z) open)

DL 02
Dom D1 flight
Actual Availability: J5 C4 D4 Z3
Hidden Availability: OY0
Website indicator: Global Upgrade Certificate Available
Actual decode: (0 RUC (OY) open, 3 GUC (Z) open)

DL 03
Dom D1 flight
Actual Availability: J3 C2 D2 Z0
Hidden Availability: OY0
Website indicator: No flag - neither is available GUC (Z0) and RUC (OY0)

DL 04
Dom First flight
Actual Availability: J3 C2 D2 Z1
Hidden Availability: n/a
Website indicator: Upgrade Certificate Available
Actual decode: (1 RUC (Z) open, 3 GUC (J) open)

DL 05
Dom First flight
Actual Availability: J3 C2 D2 Z0
Hidden Availability: n/a
Website indicator: Global Upgrade Certificate Available
Actual decode: (0 RUC (Z) open, 3 GUC (J) open)

DL 06
Dom First flight
Actual Availability: J0 C0 D0 Z0
Hidden Availability: n/a
Website indicator: No flag - neither is available
Actual decode: (0 RUC (Z) open, 0 GUC (J) open)

Other Upgrade Rules (post 01 Feb 2022)

Using GUCs to upgrade companions:

• "There is no limit to the number of travel companions you can upgrade with an Upgrade Certificate — even if you don’t need to use one for yourself.* In order to upgrade your companions, they must be flying in the same reservation or a linked reservation and each companion will require their own Certificate to be upgraded."
• *but if you are using an AMEX Companion Cert then you must use GUCs for both passengers or none (must travel in same cabin)

__________________________________________________ __________________________________________________ _______________________
---Below Text is from GUC rules PRIOR to 2/1/2022. Some info is still valid, other points need to be updated over coming days---- (utskicat 2/1/22)
** as we update these notes, I suggest the ones that remain get moved up above this section

Using Delta Global Upgrades

DL page: https://www.delta.com/us/en/skymiles...llion-upgrades
(click or scroll to "Upgrade Certificates Overview" and expand the "Show Additional Details & Terms" sections to see T&C, especially for the specific conditions governing use on partner airlines)

"Global Upgrade Certificates, available exclusively for Diamond Medallion Members, offer upgrades on both international and domestic flights with Delta, and select flights with KLM, Air France, Virgin Atlantic, Virgin Australia, Korean Air and Aeromexico."

Delta offers Diamond Medallion members the benefit of up to 4 Global Upgrade Certificates (GUCs) per year, which each allow a one-way upgrade to business class/Delta One for one person on any route in the Delta network, as well as a limited number of partner flights under specific restricted circumstances described below. 4 GUCs is enough to upgrade the Diamond Medallion member on 2 round trips or 4 one-way trips, or the Diamond Medallion member plus one companion on 1 round trip or 2 one-way trips.

Choosing upgrades as a choice benefit

• Upgrades are valid one year from the date of issue, usually the day of or day after they are selected in Choice Benefits -- so best to wait as long as possible to select them unless you have a specific use planned for them
• All Diamond Medallions also receive a Platinum Medallion choice benefit, even if just renewing existing Diamond status in a year, so DMs could for example select 4 RUCs as their Platinum choice benefit and 4 GUCs as one of their Diamond choice benefit, or 4 RUCs as their Platinum choice benefit and 8 more RUCs as one of their Diamond choice benefits
• However, it is not possible to select 8 RUCs as one Diamond choice benefit and 4 GUCs as another, nor select 4 GUCs more than once for a single year's Diamond choice benefits -- so the most GUCs anyone can currently earn in a single year is 4
• You may select your DM benefits at different times, so it is best to select fixed-expiration-date benefits (like gifting status or SkyClub memberships) as soon as possible, but wait until you have a specific use in mind to select upgrades, so that the expiration date is as far into the future as possible. This is especially important if you are waitlisting upgrades for a particular trip, since if the waitlist doesn't clear, a later expiration date will maximize the amount of time you have to try again to use them on a different trip.
• For status earned based on flying in 2017, upgrades can be selected until the last day that status is valid, January 31, 2019, so they could possibly expire as late as January ~30, 2020 if they are selected as late as possible.
• Upgrades can be used after you are no longer a Diamond Medallion as long as the upgrades are still valid, though non-DM-line phone agents will be less familiar with them, so they may be harder to use or run a higher risk of being processed incorrectly, so it may be worth booking your GUC flights before your DM status expires if possible.
• All flights to be upgraded must be flown, not just requested, by the expiration date of the upgrade certificates, though there have been some examples of short extensions offered by phone agents (for example, to the end of the month of the expiration date) as long as the upgrade is able to be confirmed before the certificates expire. In this case it is important to push the agent to make sure the ticket is reissued right away, since it will not be possible to reissue the ticket after the GUC expires. You can tell if the ticket has been reissued because the ticket number on the booking will change from what it was when you originally booked in coach, and the GUC will be marked as "closed" in your wallet on Delta.com.

Finding upgrade space

• Delta phone agents and the @Delta Twitter account can check for available upgrade space
• There is no general way to check for available upgrade space online
• However there is one exception where some flights appear to be able to be checked online: for Delta flights carrying a KLM code -- for example, most flights between the US and Europe -- ExpertFlyer.com (paid membership required) shows KLM upgrade space, and will show upgrade space for those KL-coded, DL-operated flights, which based on anecdotal evidence seems to often (but not always) correspond to upgrade space made available by Delta. This can be a way to check many dates and flights easily, but confirm with Delta that the upgrade space does in fact exist before making firm plans.
• Upgrade space is not necessarily correlated with award space
• If upgrade space is not available on your desired flight, you can waitlist, and it is possible the upgrade may clear before departure. If not your name should appear on the upgrade list at the gate. See next section for details.
• Upgrade space is rarely seen in advance on busy, high-upgrade-demand routes like ATL-JNB and LAX-SYD.

Requesting an upgrade

• You must call or Tweet at Delta to request an upgrade, they cannot be requested on the website
• If upgrade space is available, the agent will process the upgrade immediately and you will be able to select business class seats
• If the upgrade is confirmed in advance, your ticket will be reissued in business class at some point before the flight and you will receive a new itinerary email when this happens; occasionally the ticket does not get reissued and you will not be able to check in, but a quick call to Delta usually fixes this easily
• Occasionally an email is sent to alert you if a waitlisted upgrade has cleared, but often the best indication is if a segment of your itinerary disappears entirely from Delta.com, which usually means the upgrade has cleared on that segment. Call Delta to have them process it so that it reappears in your itinerary, and you will be able to select business class seats.
• There have been reports of upgrades coded incorrectly by agents -- unfortunately there is no way to know for sure if your upgrade is coded correctly, and requested upgrades seem to show up a few different ways on Delta.com, sometimes with an "upgrade requested" designation on the flight, and/or with a "phantom" duplicate flight in the seat maps which will not let you select seats
• If an upgrade has not cleared by check-in time and you do not appear on the upgrade list on the Delta app or Delta.com, call to have an agent correct the upgrade
• In certain airports where other airlines handle Delta's flights (particularly AMS), it is not officially possible to clear at the gate; KLM does process upgrades at the gate in AMS for certificates waitlisted 24 hours in advance of departure, but this is not an officially guaranteed benefit. To be safe make sure you have your GUC certificate numbers and redemption codes written down in case the KLM agents need this information.
• If you are connecting, for example flying MCO-ATL-LHR, and upgrade space is not available on ATL-LHR, best practice is to waitlist on the ATL-LHR leg only, so as not to waste a global upgrade on the short MCO-ATL flight, and then add the upgrade request to MCO-ATL when/if ATL-LHR clears; most agents will understand this approach and may even proactively suggest it
• If you have selected your choice benefits very recently (within the last 24-48 hours), the phone agent may not yet be able to see your upgrade codes, but you can read the codes to the agent from the My Wallet page on Delta.com
• The priority order of GUCs, RUCs, mileage upgrades, and complimentary upgrades is not 100% clear, but it is known that GUCs and RUCs have higher priority than complimentary upgrades, and that GUCs and RUCs appear to have equal priority. It is listed on Delta.com that GUCs and RUCs clear first by medallion level and then by time of request, which is different from the upgrade priority used for complimentary upgrades (which is also listed on the website)

Other upgrade rules

• Upgrades are valid on Delta flights on all economy fares except Basic Economy (E)
• It is possible to request upgrades in one direction of a roundtrip only, for example if you had 4 US-Europe trips planned and only 4 GUCs, you might decide to upgrade the eastbound flights only as they are overnight flights and remain in economy for the westbound flights.
• On flights operated with a Premium Select cabin, Delta has indicated that it will still be possible to use a GUC to upgrade directly from economy to Premium Select or to Delta One using the same number of GUCs. However, if an upgrade is cleared from economy to Premium Select, the GUC will be considered used, and another GUC would be required to upgrade again to Delta One. The best strategy is probably to waitlist only for Delta One upgrades, and perhaps if the GUC is close to expiring or you don't expect to have another use for it, to add a request for a Premium Select upgrade close to departure if the Delta One upgrade hasn't yet cleared and/or the Delta One seat map looks very full.
• Unlike AA and UA upgrades, DL upgrades cannot be gifted to someone whom the SkyMiles member is not traveling with

Using upgrades on KLM

Dedicated thread: https://www.flyertalk.com/forum/delt...grade-klm.html

• "For travel on KLM-operated, Delta-marketed flights this certificate may be used to upgrade on published fares booked in Y, B, M, H, Q, K, L, U, T, X or V class and all fare classes upgrade into Z class. For travel on KLM-operated flights marketed by KLM, fares booked in Y, B, M, K, H, L, Q, T, N, R, or V class are eligible to upgrade into Z class. For travel on KLM-operated flights marketed by AF, this certificate may be used to upgrade on published fares booked in Y, B, M, K, H, L, Q, T, N, R, or V class into Z class. "
• NOTE: some of the combinations above are only (relatively) recently eligible for GUCs. Phone agents may not be familiar with the new rules, and may need to be gently pointed in the direction of the DL T&C on their own website.
• In the course of processing the upgrade of a Delta-marketed, KLM-operated flight, Delta reissues the KLM flights on KLM flight numbers when they issue the upgrade, but the flights must be bought on DL flight numbers initially (unless buying expensive Y, B, or M fares with KLM)
• It is not possible to waitlist for upgrade on KLM flights, nor to go to the airport upgrade list, so space must be available in advance in order for the upgrade to clear. Many agents will offer to put you on the waitlist, but putting you on the upgrade waitlist for a KLM flight will simply never clear so you're better off monitoring it yourself (see next bullets).
• KLM upgrade space can be checked on ExpertFlyer.com (requires a paid membership). ExpertFlyer also allows users to set alerts for particular fare classes becoming available, which could be useful in lieu of being able to waitlist -- you can set an ExpertFlyer alert for "Z" availability and then call Delta to grab the upgrade if you receive an ExpertFlyer alert that it becomes available.
• KLM upgrade space on a single segment is different if you search that segment alone vs. if you search a connecting itinerary involving that segment. So if you are looking for KLM upgrades ORD-AMS-JNB, you need to search ORD-JNB in ExpertFlyer to see the true availability; even if there is space on ORD-AMS and AMS-JNB, if it Z space doesn't show up for ORD-JNB you won't be able to upgrade. Unfortunately this means that ExpertFlyer availability alerts, which can only be set on a segment-by-segment basis, are not useful for monitoring KLM upgrade availability on connecting flights.
• In some cases, adding a DL domestic connecting flight can change KLM upgrade availability. For example ATL-AMS on KLM may show availability but MCO-ATL-AMS may not. Example. Unfortunately some more complex routings, such as forcing an extra connection to find KLM upgrade space (flying JFK-ORD-AMS-CPT instead of JFK-AMS-CPT), do not show up on ExpertFlyer so you are at the mercy of hopefully finding Delta agents who know what they're doing.
• Bookings with KLM ticket numbers (starting in 074) can be upgraded using the same rules as bookings with Delta ticket numbers (starting in 006). Delta-operated, KLM-coded flights can be upgraded under the same rules as any other KLM-coded flight. Delta-coded, Delta-operated flights booked on a KLM ticket number can be upgraded the same as if they were booked on a Delta ticket number, so in any fare class other than E.
• There have been numerous challenges with KLM upgrades, so be careful, be persistent, and make sure you request the upgrade within the no-risk cancellation window of the ticket so you can cancel if it doesn't work as expected.
• More KLM upgrade reading: 1, 2, 3, 4, 5, 6

Using upgrades on Virgin Atlantic

3-year-old thread: https://www.flyertalk.com/forum/delt...lantic-vs.html
related, upgrading VS with SkyMiles (also requires G class, so the info may be useful too?): https://www.flyertalk.com/forum/delt...pgrade-vs.html

• "Valid on Delta-marketed (DL flight number), Virgin Atlantic-operated flights for use globally on published fares booked in W, Y, B, M, H, Q, K, L, U, T, X or V class to G class (Upper Class). " Upgrading on Virgin-marketed (VS flight number) flights using GUCs is not possible.
• Global Upgrades on Virgin Atlantic flights must be confirmed 24 hours prior to departure.
• Global Upgrade Certificates may not be used to waitlist for a Virgin Atlantic-operated flight. Many agents will offer to put you on the waitlist, but putting you on the upgrade waitlist for a Virgin Atlantic flight will simply never clear so you're better off monitoring it yourself (see next bullets).
• DL flight numbers are not available for most Virgin Atlantic flights to regions other than the US (for example LHR-JNB), so it is not possible to use GUCs on those flights.
• Virgin Atlantic upgrade space appears to use the same availability as awards, so if a business class award is available for a Virgin Atlantic flight on Delta.com, it should be available for an upgrade (as long as that flight is booked on a DL flight number). If an award is not available, an upgrade is probably not available -- but if you notice any exceptions to this rule please share in the thread. You can also monitor G space using ExpertFlyer (see KLM section above).
• Because G is the same code as Virgin Atlantic uses for award tickets, after you fly the tickets will register with Delta as having been awards and will not earn any miles. Watch your mileage earning carefully and you will likely have to call Delta and explain that it wasn't an award ticket but an upgrade from a paid coach ticket, and they will correct the mileage earning to be equivalent to what you would have earned on your original coach ticket.

Using upgrades on Virgin Australia

Valid on Delta-marketed, Virgin Australia-operated flights on published fares booked in Y, B, M, H, Q, K, L, U class to Virgin Australia’s Business Class.

Using upgrades on Air France

Dedicated thread: https://www.flyertalk.com/forum/delt...20-onward.html

"For travel on Air France-operated flights marketed by Air France and booked in Y, B, M, K, H, L, Q, T, N, R, or V class, customers can upgrade into A class (Premium Economy) or Z class (Business Class). For Air France-operated flights marketed by KLM, customers with a published fare booked in Y, B, M, K, H, L, Q, T, N, R, or V class can upgrade into Z class (Business Class). For Air France-operated flights marketed by Delta, customers with a published fare booked in Y, B, M, H, Q, K, L, U, T, X or V class can upgrade into A class (Premium Economy) or Z class (Business Class). In addition, customers traveling on an Air France-marketed and operated flight and booked in W, S or A class as well as customers traveling on an Air France-operated, Delta-marketed flight and booked in P, A, or G class can upgrade to Z class (Business Class)"

NOTE: GUC usage on AF used to be restricted to high fare classes only. Phone agents may not be familiar with the new rules, and may need to be gently pointed in the direction of the DL T&C on their own website.

Using upgrades on Korean (restricted to certain fare classes only)

Dedicated thread: https://www.flyertalk.com/forum/delt...ed-thread.html

"Valid on Delta-marketed, Korean Air-operated flights for use globally on Delta's published fares booked in Y or B class to Korean Air's Z class, or booked in M class to Korean Air's O class."

GUC usage on Korean requires the purchase of high fare classes, which may mean that it is cheaper to buy a discounted business class ticket than to buy an eligible coach fare and try to upgrade. Uniquely, it is possible to use GUCs to upgrade from business to first on Korean, but only if you buy the most expensive business class fares.

Using upgrades on Aeromexico

"For travel on Aeromexico operated, Delta marketed flights this certificate may be used to upgrade on Delta`s published fares booked in W, Y, B, M, H, Q, K, L, U, T, X or V class and all fare classes upgrade into Aeromexico`s O class."