I just got a new Moto Z3 upgradeable cell phone. It currently will only work on 4G but after the first of the year, an adapter(called a Moto Mod) will be available to make the phone 5G compatable,
This comes as an additional back cover that snaps on and includes an additional battery. No pricing on the modification cover yet but the Verizon salesman said it will probably be less than $80,00
As of now, this is the only cell phone that is available that can be upgraded buy the owner. Reviews are good, though it is not cutting edge performance wise. https://www.verizonwire