If you want to upgrade from CentOS 7.x to 8, you should know that you'll be on your own. As far as I know, there are no instructions out yet on how to do an in-place upgrade. On RHEL, in-place upgrades are supported. Your best move will be to back up your data, take an applications inventory, do a fresh install of CentOS 8, and then port your data and applications over.