Depends on the fare rules, but typically for roundtrip international if you cancel the return after flying the outbound you will not receive a trip credit for the cancelled portion, but rather a flight credit good for rebooking a similar return itinerary.

Say your US-MEX roundtrip was $1k while a one-way was $750. You cancel the return after flying the outbound. To use that credit for anything else besides another return from MEX-US, AA would reprice your already flown outbound to the $750 one-way leaving you with a $250 credit to use on anything.

But if you just want to change dates of the return portion, or fly to a different destination in the US, then you'd have the full $500 credit portion to use, per your original fare conditions.

If you want to change dates of the return, then there shouldn't be much additional $$ required if the same inventory is available and original fare conditions are met. Some fares say the return must start within X days of the outbound, so if changing dates pushes your return past X days then it's repriced to whatever new fare is available.

You will have to call AA and ask regardless. They can tell you exactly what the fare difference will be if you want to change dates.