No, 5G will not replace 4G LTE. The two coexist to help one another.  If you’re in a 5G-supported area and have a 5G-capable phone, you will automatically connect to the 5G network. If you happen to be in an area where 5G network is still under construction or not available, you’ll automatically have access to the 4G LTE high-speed network you’ve always used.