Just to clarify your only option is same day flight changes which will carry a cost of $75, if available. You may start the process 24 hours before your scheduled flight. E inventory will need to be available if you in economy. This can change throughout the 24 hour window so check back throughout the day and evening prior. It can be done on-line.

https://www.aa.com/i18n/utility/sameDayTravel.jsp