This wiki was created to distill SDC (and related standby) down to a set of rules in the hopes of benefiting others.

The official "rules" can be found here. In practice, some of the rules almost always apply, and some are more flexible.

Present Eligiblity policy 2023
Note there have been reports of SDC to one segment removing confirmed CPU upgrades on downline segments. If maintaining the confirmed upgrade is important then these changes should be handled with care.

Most Frequently Asked Question: When do I call/go online to make the change?
Follow this flowchart:
Question 1: Are you within 24 hours of your currently scheduled UA(*) operated flight that you're trying to change off of? If yes, continue to Question 2. If no, continue to answer 1.
Question 2: Are you within 24 hours of the first UA operated flight you're trying to change on to? If yes, continue to Answer 2. If no, continue to Answer 1.
Answer 1: No, you cannot call/go online yet to make an SDC.
Answer 2: Yes, then you can call/go online to make an SDC.

General Baseline Rules

These are the general rules that usually apply to SDC's; falling outside these rules usually requires payment of a full change fee and the fare difference:

• Not available for Basic Economy (N class) tickets
• The departure time of the first segment of the old itinerary and the departure time of the first segment of the new itinerary must both be within 24 hours of the moment that the change is made.
• Only applies to UA/UX operated flights, domestic or international. Changing from a non-UA operated to UA operated may be possible.
• Must be issued on UA ticket stock (ticket number begins with 016)
• Award tickets are eligible, but an award change may be a better option (not restricted to 24 hours)
• The ultimate origin and destination airports must remain the same.
• Routing can be changed (addition, deletion, or change of connection points) but the routing rules of the original fare usually apply.
• All remaining unflown segments in a single fared itinerary are considered together (i.e., if you're flying WWW-XXX-YYY-ZZZ, you can go to a kiosk at XXX and potentially fly XXX-AAA-ZZZ or XXX-ZZZ direct).
• SDC can be done an unlimited number of times, and can, therefore, be used to extend a trip indefinitely (pending availability).
• SDC costs $75 for non-Premiers Only premier members as of 1 Jan 2021
• No fee for Premier (plus any applicable fare difference, see below). *G's may also get free SDC, although there is no mention of this in the official rules. Once in-route, fees for an earlier connecting flight may be waived per GG SDC Line 118.
• Non-elites on the same PNR benefit from any waived fees for SDC.
• No fare difference is charged so long as there is availability on all of the new segments in the originally ticketed fare class. Fare class availability in all classes often opens up after T-24 and almost always opens up at T-3 unless the plane is very full. The practice of fare leveling all fares for all flights at a specific time appears to no longer be in effect.
• SDC on bulk tickets, including United Specials, is not permitted.
• SDC is permitted on both domestic and international itineraries.

How to SDC

SDC can be accomplished in a few different places (in rough order of likelihood of being able to get the rules bent):

• United mobile app
• Kiosk
• Phone
• On united.com, right now only the "Search Other Flight Options" button during On-Line Check-In (OLCI) works. This has spotty availability, particularly if the change would involve paying an additional fare due to fare bucket non-availability.
• Counter/Gate (although reports are that most airport agents are not familiar with the SDC policy and may not allow you to SDC to a valid itinerary--you can try to direct them to look up GG SDC, but they often won't understand that what you're trying to do is permitted by those rules)

Please note that the United.com “change flight” link before checking in within T-24 is broken. The website may not show all available options and will often attempt to collect the change fee rather than the SDC fee (if any).

NB: An SDC where the original itinerary (in one direction) has three or more segments cannot be done online.

Nuances and Loose Rules

Now for the hard part: figuring out where the flexibility is in the rules. This list is compiled from the responses to people saying, "Can I do X?" where X is something that is not clearly permitted by the rules. One's likelihood of success here depends a bit on the method used for the SDC.

For each item, the following codes apply:
Y = officially permitted and possible using any method
A = not permitted but perhaps possible using an agent (phone/counter), perhaps having to HUACA
A* = mixed reports of success with an agent (phone/counter),
N* = possible only if you find a very accommodating agent
N = little or no reported success.

• Violating various non-routing fare rules (day/time, minimum stay, etc.): Y
• Change destination to ultra-close airport (see the old S*FAR/STANDBY): A
• Change destination to a co-terminal: N*
• Change destination to an entirely different place: N
• Change itinerary at the beginning to include illegal layovers (> 4 hours): N*
• Change itinerary at the layover point (using kiosk, app, or agent) to create an otherwise illegal layover: Y
• Change itinerary at the beginning to add a layover point not permitted by the routing rules: N*
• Change itinerary at a layover point to add another layover not permitted (either in terms of number of segments or routing rules) by the routing rules: Y
• Change to a flight exactly 24 hours before or after the original flight: N*
• Change to a flight exactly 24 hours before the original flight, if the previous day's flight has been delayed: Y (based on one report)
• Change to a flight exactly 24 hours after the original flight, if the original flight has been delayed: Y (at least in the app)
• Combining separately-fared segments to do SDC (e.g. XXX-YYY+YYY-ZZZ -> XXX-ZZZ): A
• Change UA-codeshare, non-UA-metal segment to UA metal: A*
• Change from UA metal to UA-codeshare, non-UA-metal segment: N (see post 1813 for one exception)
• Change from UA-codeshare, non-UA-metal segment to a different UA-codeshare, non-UA-metal segment: ??
• Change on a non-UA ticketed segment: N* (with potentially dire consequences)
• Change from an upgraded flight into Y on the new flight: Y
• Change from a PointsPlus-upgraded flight into J/F on the new flight: A
• Change from a CPU-upgraded flight and get confirmed into available PZ space on the new flight: N*
• Change from an instrument-upgraded flight and get confirmed into available PZ space on the new flight: A*
• Non-elite on different PNR benefiting from Elite's waived-fee SDC: A
• Change at origin airport after tendering a checked bag: A - 60 minutes prior to both flights required
• Change at layover airport when a bag has been checked: A - 60 minutes prior to both flights required
• Change a bulk fare: A*
• Stand-By to later flight, then SDC after original flight departs to flight within 24-hours of new Stand-By flight: Y

If you encounter issues with an agent (phone or counter/gate), you can ask the agent to reference GG SDC. Additionally, GG SDC line 61 specifically references that fare classes are supposed to level out 3.5 hours before departure, so you might be able to use that to your advantage if fare classes have not leveled out and you're trying to ask an agent to open up the correct inventory or force you onto the flight.

Note -- as of 11/27/2017: A datapoint indicates that UA may have updated its GG rule regarding leveling out to state the following:

Code:

EVERYDAY AWARD AND REVENUE BUCKETS ARE TO LEVEL OUT WITHIN 2H OF DEPARTURE

The practice of fare leveling all fares for all flights at a specific time appears no longer in effective -- it does happen in some case but appears to be more variable

Standby
For all fare classes other than Basic Economy (N class) you may standby if seats are not available in the purchased fare class. There is no longer a fee for Standby. In these cases, the same-day change fee will apply, but will not be charged unless you are assigned a seat on your alternate flight. Changes in routing are not allowed when standing by. Standby may be requested up to 24 hours of original departure on the day of departure at an airport kiosk or with a United representative at the airport or UA app. Once you are added to the flight standby list, you can check your real-time standby status at united.com/flightstatus or on your mobile device at mobile.united.com/info.

Instead of paying a same-day change discounted fee, can I stand by for my desired flight for free?
Yes No. The only time you may standby for free is when the fare does not have any penalties for making changes, or when standing by involuntarily due to a flight irregularity.

Can I pay the same-day change discount fee with cash?
Cash will only be accepted by a United airport representative when the change is available to be confirmed, not for standby travel.

Standby Nuances and Loose Rules

All nuances assume request is made within 24 hours of scheduled departure to a flight within 24 hours of the request.

For each item, the following codes apply:
Y = officially permitted and possible using any method
K = Perhaps not officially permitted, but kiosk will allow.
A = not permitted but perhaps possible using an agent (phone/counter), perhaps having to HUACA
A* = mixed reports of success with an agent (phone/counter),
N* = possible only if you find a very accommodating agent
N = little or no reported success.

• To a flight the previous calendar day: A*/N*/K (app allows this)
• To a flight after your ticketed flight, on the same calendar day: Y
• To a flight the following calendar day: N*
• Request Standby at an airport other than departure airport: Can now request Standby via app 24 hours in advance
• Make request calendar day before scheduled flight to flight same day as scheduled departure: K

Mileage Credit after SDC
SDC is considered a voluntary change, so your credited PQM & PQS will be credited based on the route you actually fly. Your PQDs should not change and therefore for your RDMs will not change. Occasionally if you fly a fewer number of segments, your PQD will credit wrong, but a call to MPSC should correct that.

Previous threads:
UA Same-Day Travel Change (SDC) Fees/Questions (with Wiki) {Archive}
UA Same-Day Travel Change (SDC) Fees/Questions *now with WikiPost* {pre-2015 Archive}