FFC == "Future Flight Credit"

When re-booking 3-March 2020 purchased tickets, if the new ticket is cheaper you will receive an ETC (good for 24 months); starting April 2021, this is now a new form of FFC
However for tickets purchased 3 March or later, the fare rules now state there is no "residual value" meaning no ETC for the fare drop and if you have to rebook again the credit is just the value of the new ticket.
For pre-1 April 2020 purchased, FFCs will be good for 24 months from the original purchase date
And there are no change fees
Change Fees Are Gone For Good

For purchases starting 1-April, the lifetime of FFCs are returning to the traditional 12 months from purchase

In some cases of voluntary cancellations, FFCs may be convertible to ETCs (seems pre 1 August 2020 purchase is a factor) but this can only be requested on the phone and may take days/weeks. This may be a way to avoid the problem of "no residual credit" YMMV


Update 4 April 2020
The below needs to be updated based on the changes above

This wiki discusses the for steps in using the "future flight credit" from a canceled non-refundable ticket. Note this is for the cancellation of an entire ticket -- the process is different for a partial flown ticket

- Note some tickets are non-changeable and have no residual value if not flown:
United's Basic Economy - Discussion, Q&A, ...
New UA/*A TATL -LGT Economy fare - first bag charged, no changes allowed

- Notify UA of the cancellation before the scheduled departure. An absolute must, if not done the entire value will be lost. But see note below about Travel Waivers

- Wait until you are ready to reschedule. When ready you will need to pay a change fee in new money - the fee varies and will depend on the fare rules of the original fare. Note in many cases there is no longer a change fee (BE and flights origining outside the USA may still have a fee)

• The new ticket must be purchased within 1 years of the original ticket's purchase date (pre-April 2020 purchases receive 2 year FFC)
• The new ticket will be good for 1 year from the exchange/re-scheduled date and travel must commence prior to the expiration of the original ticket (an exception is if the new travel is just a date change, in this case, a new ticket may not be issued and initial purchase date will still prevail)
• The new ticket must be for the same named traveler
• The portion of a ticket purchased with nonrefundable ticket credit will be nonrefundable regardless of the new ticket's fare rules
• The change fee requires payments of new funds and cannot be funded from the original ticket's value
• Any fare residual will issued as an ETC valid for one two year.

- If an ETC is issued, all of the funds in it must be used within one twos year of issuance.

• The ETC can be used by anyone.
• The one two-year limitation only limits purchasing, not travel - travel may be outside the 1 2-year limit.
• There are no additional fees to use the ETC
• The credit voucher can only be used for UA/UX operated flights (workaround - UA Electronic Travel Certificate & Codeshares/partner flights)

To get maximum ETC (or new type FFC), an option is to pay the change fee (if any) and book the cheapest possible OW fare and throw that ticket away -- how to find that cheap fare, see Cheapest possible flight (How do I redeem this flight credit?)

If the reason for the change / cancellation is due to medical issues or death of a close relative, see Consolidated "Refunds/Cancellations Due to Illness/Death" Thread [Merged] for the process to get a refund of the change / redeposit fee.

Other cases for "free" changes are:
1) if the flight is covered on the day of departure by a waiver (WX related or other reasons). Note there may be a limited time frame for the change fee waiver.
2) or by an airline's change in schedule

What if it is 1 year from the original ticket purchase date and I am not ready to buy another United ticket using my canceled ticket's funds? Buy a throwaway ticket on the cheapest fare you can find. United will issue an Electronic Travel Certificate for any unused funds. That ETC is valid for one year from issuance (see https://www.flyertalk.com/forum/united-airlines-mileageplus/1369495-ua-electronic-travel-certificates-etc-general-q-new-combinability-policy-34.html). If you do this on the expiry date of your original ticket, you effectively extend the availability of the funds for an additional year at the cost of a throwaway ticket.

What if I purchase a refundable ticket with the credit from the non-refundable ticket, can I then cancel and get a refund? No, the funds from a non-refundable purchase remain non-refundable even if used to purchase a refundable fare.

Cannot find the canceled reservation online?
Go to Manage Reservations and look in both "Current" and "Cancelled" tabs
Find the appropriate reservation and look for "Use Future Flight Credit"

After rebooking a canceled ticket, is there a new 24-hour window for fee changes?
No, the 24-hour flexible change only applies to the original /new booking and does not extend to rebooking of canceled / changed tickets.
United’s 24-hour cancellation / flexible booking policy

What if one leg is a refundable fare and another leg is non-refundable?
For a single ticket, the most restrictive fare rule applies to the entire ticket. So a refundable segment paired with a non-refundable segment makes the entire ticket non-refundable. Same with the change fee, the segment with the highest change fee applies to the entire ticket.

Archive thread: How to get residual/"future flight credit" from non-refundable flight {Archive}

Other related threads
Changing/Canceling/Replacing a ticket costing less than the change fee?