AA lets you cancel a paid itinerary before flying the first leg, and use the residual (after deducting a change fee) for the purchase of any other ticket, either then or in the future, within one year of the original ticketing.
I can't quite articulate what happens after you fly the first leg... but they more or less corral you into a similar itinerary with same inventory requirements, or a long and unpredictable time on hold while they are off at the rate desk. I can tell you that AA's handling of this is FAR MORE FAIR to the ticket purchaser than Delta's is. I have a pile of useless unused Delta return tickets to prove it.