though airport control / check in / lounge staff may try to change you a earlier flight without a fee (tried it a few times before for busy route like BKK and TPE), only provided that your ticket is a CX ticket
Tried one time on a BA issued redemption ticket on CX metal to TPE, but was asked to call the BA service centre as the ticket was not issued by CX and so the check in staff / airport control cannot change the ticket and move to an earlier flight
so i guess if your ticket is issued by AA but on a CX metal, you will have to call the AA reservations.