USA will have 5G by 2021 so I am afraid all in USA will have a very little time to play with their pyras only if ????
5G is not a replacement for 4G. 5G has half the coverage (so you need a lot more cell towers) and the bandwidth of 4G isn't really used at the moment anyways.
5G is useful for low-power IoT devices, as the power consumption is a lot lower.
So I doubt companies will switch off 4G anytime soon. It wouldn't make any sense.