I've never done it. I'm not sure it's formally supported, but it is formally documented, and it looks like a fairly common thing to do. Look at http://wiki.centos.org/HowTos/MigrationGuide , in the section entitled "Migrate an existing system from RHEL6 or SL6 to CentOS 6" (or the section immediately above it for EL5 to C5).
And if any future readers find this question, my personal feeling is the best thing you can learn from it is don't install RHEL unless you really need it, and if you decide you do, make sure you can afford it for the life of the system. I've yet to meet a system that really needs to be running RHEL, unless it's also running some piece of FascistWare that checks the version and vendor of the OS before permitting itself to run.