I am pretty sure that you are going to have to pay off your AT&T NEXT device(s) before attempting to buy a new iPhone; I don't think it is possible to go to Apple and attempt to trade in a device which isn't fully paid off to AT&T. They -- Apple and AT&T -- are two separate companies and although they do work together this kind of scenario probably is not acceptable to either party. In previous years I have used the NEXT program to purchase my iPhones and then in the next autumn when a new iPhone is on the horizon have paid off the balance owed to AT&T prior to then going to the Apple store to trade in the older device, purchase the new iPhone and begin a new NEXT program with the new iPhone. However, I have read on MR that now AT&T has somewhat changed their NEXT program and so I am considering simply this time paying off my iPhone XS as usual and then either going with the Apple program or paying for the new phone outright from the get-go. In any case whatever I do I will be trading in an iPhone and getting credit for that, which does help with the financials.