Originally Posted by JJeffrey

All tickets are valid for 1 year from the date of issue, so they were correct in that you cannot keep extending the date out of the same ticket past 1 year.

But you can definitely change to a cheap flight before it expires and get the remainder in another voucher, which will be good for another year.

Right now there are a number of dates in June where MIA-TPA is $39 one way. Switch your current flight to that and AA will issue you a new voucher for $161 good for another year.