I am two payments away from having mine and my wife's iPhone X's paid off with the AT&T Next year program. We'll be leaving it for good and going to Apples upgrade program. On AT&T's program, when you upgrade, you are responsible for mailing your trade-in back to them. They won't take them in store. I did everything they asked me to do and had my trade-ins mailed back to them the very next day. It took three month's before they said that they had received my trade-in's so they kept charging me my monthly fee for my old traded-in phones while also charging me for my new phones. This occurred even though i was able to see via the tracking information that they had received my trade-in's within a weeks time of me sending them to them.
Their explanation was that sometimes there is a lag in their computer system. It then took another three months before they refunded my payments that they had charged me on my old traded in phones. That's why I am still on an iPhone X. I planned on paying these off, owning the phones outright to possibly hand down to my children or other family members, and switch to Apples upgrade program. AT&T's program is too much of a hassle.