American Airlines charges $200 to have a domestic ticket re-issued at a lower available fare. So if the price of your flight drops in excess of $200, you can obtain a "rollover" credit for future travel on American Airlines. Similarly, American Airlines charges up to $250 to have an international ticket re-issued at a lower available fare.
To obtain the refund you must call American Airlines Reservations: 1-800-433-7300.
American Airlines' policy refers to travel credits as "rollover" credits. A "rollover" credit would be the refund a traveler could receive for the difference between the original purchase price and the new lower fare. If you've already purchased an American Airlines flight - and that flight is subsequently available at a lower available fare that's net of the Airline's change fee, you can get a rollover credit good towards a future flight on American Airlines.