John R Pierce wrote:
see Migration from RHEL5 to CentOS5 near the bottom of http://wiki.centos.org/HowTos/MigrationGuide
if you have 4, its similar but different. if you have 3, time to wipe and upgrade IMHO.
oops, eat my words, here's RHEL3
http://wiki.centos.org/FAQ/CentOS3#head-10bce23c2383ab4be8a9f0926578e96f5e0a...
IIRC, the procedure for RHEL4->CentOS4 is somewhere between the 3 and 5 process. you need to install yum and the repo files as well as the centos keys, then do the rest ...
note all these procedures will result in a hybrid system where some of your packages are from the upstream vendor, and others from the centos project. while this SHOULD work together OK, and many of us have done exactly that, it is officially UNTESTED and you're on your own.