Is it possible to early upgrade phone under old promo?

I got my iPhone 16 Pro through the “on us if you upgrade plan” promo, and I’m still receiving credits toward it. If I upgrade early to the iPhone 17 Pro, what happens to the remaining balance on the old phone? Do the credits stop, and would I then be responsible for paying off what’s left on the iPhone 16 Pro, or could I trade it in as part of the new upgrade? Would I lose the rest of the promotional value from my current deal once I start a new installment plan for the iPhone 17 Pro? When I apply my current phone as a trade-in for the 17 Pro, it shows no payments, just a new 24-month commitment, does that mean the trade-in clears out the balance and cancels the credits from the old promo, or am I still on the hook for something?