This is covered in the terms and conditions
https://www.aa.com/i18n/customer-ser...vel-credit.jsp
- If the ticket price is less than the value of the Flight Credit, you can only use the value of Flight Credit needed for the current booking while remaining value will be issued as travel credit for future reservations.
Surprisingly quite generous. The IT effort to retain the original expiration dates for the unused amounts must have been overwhelming.