RHEL switch
I have only done this once, with RHEL v4 onto Oracle Linux. The switch just changed my update provider, and Oracle supported the previous OS packages that had been shipped by Red Hat.
Red Hat now has a script to convert an installed CentOS or Oracle Linux to RHEL; previously a wipe and reinstall was required ("have fun reinstalling your system" is still on Oracle's CentOS site). The description looks much more thorough in replacing all possible packages with Red Hat versions:
https://access.redhat.com/articles/2360841
I don't know of any other current repo conversion options for a CentOS install, but some will likely emerge (Rocky and Scientific Linux come to mind).
Oracle's CentOS notice, with (some) recent updates:
https://linux.oracle.com/switch/centos/
New CentOS conversion blog entry:
https://blogs.oracle.com/linux/reasons-for-switching-centos-to-oracle-linux
New repo conversion script, supposedly works with CentOS 8:
https://github.com/oracle/centos2ol
After conversion from CentOS to Oracle Linux, free updates are available indefinitely. For those who desire paid support, there are several (yearly) price tiers ($119; $499; $1,199; $1,399; $2,299). I will say that I've had more problems with Oracle support this year than ever before.
For those uncomfortable with "free" products from Oracle, bear this in mind: in the last two decades, Red Hat has terminated two Linux distributions, while Oracle has terminated none.