If you upgrade at 12 months and give the iPhone back to AT&T do you have to pay the remaining payments or are they nullified by returning the phone?
Trading in the phone after a year is usually a bad choice because you've essentially leased the phone.
You make a year's worth of payments and walk away with nothing.
Might be a better choice to pay off the iPhone 6 before upgrading to the 6S and then selling the 6.