What happens to my existing EIP if I buy the new iPhone through Apple?

Hey everyone, sorry if this is a dumb question, I'm a bit confused about how this works. I have an iPhone 17 Pro Max on a T-Mobile installment plan about halfway through. I still owe around $650, and I'm getting monthly promo credits on it. I want to buy the new iPhone at the Apple Store with T-Mobile financing and trade in my 17 Pro Max to Apple (not to T-Mobile). What happens to my current installment plan? Does it just keep billing every month until it's paid off? Do I keep getting the promo credits? Also, can I even finance the new phone through Apple on T-Mobile while I still owe on the old one, or will that get blocked? Thanks!