Welcome to Android Central! If you're on the Next program, you could get a new phone in a year.
AT&T Next - Get A New Smartphone Every Year from AT&T Wireless. If you're not on Next, then it's too late to return the phone to the store for an exchange. Purchasing a new phone from AT&T would have to be full retail, which will obviously be pretty expensive. In order to sell your current phone privately, you'd have to pay it off first--otherwise, AT&T would blacklist the phone since it wasn't fully paid for.
But managers can make exceptions sometimes. It never hurts to talk with the store manager and see what your options are. If your current phone is in pristine condition and if you've been a longtime loyal customer, maybe they'd make an exception for you. But I doubt it.
