Hi All! Retired now but still running some flavor of Red Hat on my home boxes. Just ran the converter on a CentOS 9 VMware VM and a CentOS 8 install on an old Athlon box. Both appear to have gone flawlessly. I will add that both are running Mate as the desktop.
When I first saw the title of this post, I imagined it was going to be a story about updating an installation of Red Hat Linux 5 all the way up to Rocky Linux, and I was a bit horrified at the thought, but intrigued, lol.
I try to stitch in when and how I started with Linux when its appropriate in this kind of introduction. Sometimes gets me past the, “Are you sure you know what you’re doing?” type questions. No version of RH (Enterprise or not) has ever supported upgrading major versions. Not sure if I can get R9 to install on the old Athlon box so it may be stuck at 8 until the box gets retired.
One can be “a user” and never really learn about the system that they use. Just look at majority of “other OS” users. I was “just a user” until I got incentive to peek what is under the hood. Incidentally, that was about 1998 and the “hood” was RHL.
You wrote “CentOS 9” and “CentOS 8”. Those would be CentOS Stream  now. The Streams are a different beast, not bug-for-bug compatible with latest RHEL like the CentOS Linux and Rocky Linux were/are.
I’m in the habit of using my “most recently retired” hardware as a testbed for whatever new release is coming down the pipe. Usually works but I end up running into a variety of hardware compatibility issues like no longer supported video or other expansion cards, insufficient RAM, etc. Sometimes it is something a little more subtle… like this. So, looks like the old Athlon box doesn’t get upgraded beyond Rocky 8. I think I built it in 2010 so not a bad run.
I think the OS progression on that box was something like CentOS 6 → CentOS 7 → CentOS 8 → RHEL 8 (free developer license) → CentOS 8 → CentOS 8 stream → Rocky 8. My CentOS 9 stream install was as a VMware ESXi VM. Ran the migrate2rocky9.sh script and its now a Rocky 9 VM.