How do I migrate from RHEL 5 to CentOS 5?

Since CentOS strives to be as compatible as possible, that approach should work just fine. We're converting our RedHat boxes to CentOS as well, and I've simply added the CentOS repos for packages I've needed until I can do a clean move over. Most packages don't check explicitly for RedHat (at least not the ones we've needed). I'd feel safer with a fresh install though if at all possbile.

Otherwise, just follow the CentOS Migration Guide HowTo.

I would highly recommend only doing this as a temporary procedure until you can schedule a time to move to a fresh system though.

Also, make sure you're pointed to the same point release (for yum repos) as what your current RHEL is at. I ran into some odd dependency issues recently which were the result of using the mirror repos on an older point release. Had me confused for a little why things were failing.