Just upgrade. It's suggested to switch to a plan with UW 5G for the newer iPhones, but Verizon is nowhere near ready for 5G to live up to the hype they made for themselves. Any phone will work fine under your current plan.
It's great to know that you have one of our special plans, but could you share a bit more about where you are seeing that you need to change your plan to upgrade your phone? The only time a plan change would be required is if you wanted to use a promotion that had a specific data plan required, and that is always listed with the promotion's details.