When changing tickets there are two models for determining the new price ...
1. reprice everything at today's fares
2. reprice everything at the same prices as on the day you bought the original tickets. This means paying just the change fee if the same fare bucket is still available on the new flights.

Model 1 almost always applies when you change the outbound flights and model 2 almost always applies once you have flown the first flight.

If you haven't yet flown the first flight and yet you are only changing the return flights then tickets vary as to which model they use and you would have to read your terms carefully. I recently had a BA ticket that had model 2 rules for this circumstance and changing my outbound 6 days before my trip was going to cost $2000 (because of advance purchase requirements not being met), whereas changing the return cost just $275 (change fee).

Airlines don't seem to like making the rules clear or even readily available, but in situations like yours they are the place to start. If you can't find them or make sense of them, then call AA and ask them what changes can be made with re-pricing at historical fares. And then you need to find new flights that still have the same buckets available as your current flights. If you have to wait until after you have flown your first flight, then it is all a big unknown and effectively a gamble.