I am confused on this as well since I have historically used the Next installment agreement and just trade in after the 12 months. Does the credit just apply against the remaining payoff amount?
No. I have a very reliable AT&T source that is on top of these things. Once you trade the phone in, the credits go away. AT&T has a good rule of thumb for this:
If you maintain possession of the old phone, you keep the credits. If you hypothetically traded in a phone at the 12 or 15 month mark, you’d lose the remainder of the credits you are owed.
In order to truly get $800 off, you’d need to keep that 12 Pro for the entire Next agreement which are all 30 months.
Believe me or don’t, but this is 100% the way it works with AT&T.