Just did a dummy booking at AA and took a look at the terms and conditions.

NOTE: I have no idea what class you booked into, so your restrictions will likely vary.

What I found was:

FARES CHARGE USD 150.00 OR HIGHEST FEE OF ANY CHANGED FARE COMPONENT WITHIN JOURNEY

and the change can't be to the first leg - which in your case is OK.

In English, they're saying that it'll cost you a minimum of $150 to change, and if the new itinerary is more expensive the fee will be higher. (If old flight was $400 and the new is $500 you pay $150 (minimum). If old was $400 and new is $600 you pay $200.

But again, the restrictions are different for every booking class, so you really need to call AA.