Can I ask why you want to just turn in your old phone? Granted I don't know what phone you have but it might be worth more than what you owe on it, especially if it's an iPhone. In that case, pay it off, sell it and pocket the difference.
This is what it shows me if I start the upgrade process:Only in AT&T store?
Mail-in old phone?
Hopefully not! :/just had an att chat rep tell me the original box and all its contents need to be turned in. who knows if this is accurate or not