You can rebook in future. Just make sure you have the e-ticket number (NOT the record locator) to hand to quote because the only way the reservation agents have to find the credit while you are on the phone to them is to look up the previous ticket number.
For example: in Jan 2015 I booked travel for Aug 2015. My plans changed so I cancelled that trip in April 2015. Later in August 2015 I booked travel for January 2016, using the credit from the first ticket.
Travel must begin within 1 year of the original ticket purchase date. Therefore the latest I could start travel was Jan 2016. As with all (AA) tickets, travel must finish within 1 year of starting, so the furthest you can stretch this out is the last of your travel a bit less than 2 years after the first ticket purchase.