No there's no 'upgrade' window. If you are returning your 16GB for a 32GB you'll either have to go into an Apple store and swap it or order the 32GB from ATT/Apple paying the full upgrade price and wait until it arrives and receive credit for returning your old phone (within the 30 days). Not going to work the way you are thinking. Highly unlikely this can all be done within ~15 days. Best bet, go to an Apple store, sooner than later.