New phone, new plan

If you buy an iPhone 16 directly from Apple, you won't necessarily have to change your current plan with T-Mobile. You can simply insert the T-Mobile SIM card into your new iPhone 16 to keep your existing plan and number. If only one of you upgrades to the iPhone 16, it shouldn't change anything with T-Mobile as long as the new phone is compatible with T-Mobile's network . You can check compatibility by entering the IMEl number of the new phone on T-Mobile's website.