Somewhat related:
I had purchased a MSY-NYC-MSY ticket with the first leg operated by B6 and the return by AA. After taking the outbound flight this past Saturday, I decided on Sunday to change my return to Sunday rather than Monday. The app did not allow me to make the change -- I think because I had used a flight credit when I purchased that ticket, though perhaps because of the combined B6/AA itinerary -- so I had to call AA. After waiting half an hour for a callback, I immediately was disconnected, so had to get in the queue again. At this point I was worried about losing my desired flight or fare -- I was flying that same day -- so rather than wait, I just purchased the new ticket and canceled the old one, in both cases directly on the app. I knew that would generate a flight credit for the canceled flight, but it never appeared in my AA wallet.
So today I booked MSY-BTV/NYC-MSY and put it on hold. I then called AA -- today I got right through -- and explained to the agent that I wanted to purchase the new ticket using the flight credit from this past weekend's canceled flight. She said she had to put me on hold to call revenue management, but she accidentally hung up on me instead. I called back immediately and immediately got another agent on the line. While the second agent was reviewing my records, the first agent called back. I asked the second agent to remain on hold, and I'm glad I did; the first agent came on the line and said that the flight credit could not be applied since it was a B6 ticket. She said it only could be applied to another B6 fare. I politely ended the call with her, and fortunately the second agent still was on the line. She said that sounded incorrect to her; the first leg, which I actually flew, was B6, but the canceled leg was AA. She got revenue management back on the line, and then came back on the line to tell me that the credit would be applied to the new reservation, which she did.
So a bit of a pain, but it all worked out.