If you don't/can't use the voucher by the expiration date, you can renew it for another year I believe - but the fee is like 100 bucks so it's pointless if that's the value of the voucher. It also depends on why you got the voucher. If it was because you volunteered to take a bump, then standard rules apply. However, if it was issued due to an involuntary bump, where you were denied boarding because of oversales, weight restrictions, etc., I think they will work with you differently on the renewal fee.

Of course, as with any carrier, the rules change at AA in the blink of an eye and it pretty much always boils down to who you get on the phone when you call.