According to https://www.aa.com/i18n/plan-travel/...day-travel.jsp

If you bought a ticket with a partner airline for a flight operated by American, you can stand by or buy a same-day confirmed change. Use your American Airlines confirmation number to view your trip.
Starting 24 hours before departure, go to your trips online or in the app, or stop by an airport ticket counter to request a change and pay the fee, if applicable.*