In my search, I found a thread about people experimenting with extending credits just before the end of last year. The reports were inconsistent, and didn't relate to my specific question (changing/canceling flights after the ecredit used to purchase it expires). Nobody in that thread thought to look at the expiration date on a ticket's receipt booked using the ecredit, which may indicate the expiration date of the ecredit if the ticket is canceled.

I just finished booking several flights using ecredits. Here's my data points:

• Yesterday I used part of an ecredit that expires on March 28 for a roundtrip March 14-17. The expiration date on the ticket is listed as Jan 25, 2025.
• Today I used part of the same March 28 ecredit plus part of an ecredit for my wife that expires July 25 for a roundtrip for both of us Feb 21-27. The expiration on my ticket is listed as Jan 26, 2025, and her ticket was unchanged at July 25.
• Then I used the rest of my wife's July ecredit for a roundtrip Sep 29-Oct 6. The expiration on her ticket was listed as Jan 26, 2025.
• Finally, I used the remainder of my March ecredit along with 3 other ecredits for the same roundtrip Sep 29-Oct. 6. The expiration on my ticket was listed as Jan 26, 2025.

Here's the pattern I see: If the booked flight is scheduled for a date shortly before or anytime after the ecredit expiration, then the expiration of the ticket is extended. Otherwise the expiration of the ticket is the date of the ecredit expiration.

My only other question is: If I cancel a ticket, is the expiration of the resulting ecredit always the date of the ticket expiration listed on the flight receipt in the original confirmation email when I booked the ticket? That would make sense, but a couple of posts seem to doubt that's always the case.