Transfer device and trade in questions.

My husband broke his phone. I have an iPhone 15 Pro Max that we still have 12 payments left on, as well as trade in promo credit being applied. I also have an older iPhone still laying around.

Could we transfer this iPhone 15 Pro Max to his line (on the same account/bill as mine) and still continue device payments and trade in credit it’s still receiving? Then I take the older iPhone and trade it in to get the iPhone 17 Pro, iPad, and Apple Watch on Verizon deal?