The following information should allow you to do an in place upgrade of Redhat to Centos
It is safest to upgrade with in the same major version number. (eg RH 5.1 to Centos 5.8 not Centos)
**Do this at your own risk. I am not responsible if you try this and it does not work – nighthawk**
You can choose any repository that you prefer.
If you are doing this to an outdated version then you may need to goto (vault.centos.org)
Log in as root and start a terminal session.
yum clean all
rpm –import RPM-GPG-KEY-CentOS-5
rpm -e –nodeps redhat-release
rpm -e yum-rhn-plugin
rpm -Uvh –force *.rpm
If you receive dependency failures then wget the additional dependencies that are needed
yum upgrade -y