Then you misunderstand your agreement. You always have the OPTION to trade in any device, but you're not REQUIRED to trade a device; especially not for a 2-year-contract agreement. You need to check what agreement you are currently on.
On an AT&T Next plan, there are 3 time periods you can choose from for making payments (18, 24, and 36 months). The longer your agreement, the less you have to pay, but the longer you have to wait to keep the device forever.
It gets tricky from here and this is why you really need to read the website, verify your existing agreement, and speak to a representative at an AT&T store.
Let's say you chose the 24-month AT&T Next plan. AT&T will tell you that at 18-months you can trade in your device for a new one. You can do that, but you lose the device you've been paying for. At 24-months you can keep the device forever and continue to use it or buy a new phone and still keep the one you just paid off. It works kind of like a rent-to-own service.
If you are on a 2-year contract, then you really should wait until that ends before making any decisions. If AT&T is telling you that on Sept 22 you have to trade in your device, then you're on a Next plan and you have to reached your milestone to be allowed to keep the device. Of course you don't HAVE to upgrade and can choose to finish out your payment plan so you can keep your device.
But I can't verify your account information on a forum and don't want access to your info to do it. The info is available online and store reps can view your account and answer your questions.
I hope I've clarified how most of this works, but again, I'm not a rep, so there's room for outdated or misunderstood information from my viewpoint.
