This is actually a lot more simple than I was expecting. Recently our RHEL subscription for a backup server expired and couldn’t really justify the cost to renew it, so I decided to convert it to a CentOS installation by doing the below:

yum clean all
mkdir /usr/src/centos
cd /usr/src/centos
wget http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
wget http://mirror.centos.org/centos/5/os/i386/CentOS/centos-release-5-3.el5.centos.1.i386.rpm
wget http://mirror.centos.org/centos/5/os/i386/CentOS/centos-release-notes-5.3-3.i386.rpm
wget http://mirror.centos.org/centos/5/os/i386/CentOS/yum-3.2.19-18.el5.centos.noarch.rpm
wget http://mirror.centos.org/centos/5/os/i386/CentOS/yum-updatesd-0.9-2.el5.noarch.rpm
wget http://mirror.centos.org/centos/5/os/i386/CentOS/yum-fastestmirror-1.1.16-13.el5.centos.noarch.rpm
rpm –-import RPM-GPG-KEY-CentOS-5
/bin/rpm -e --nodeps redhat-release
/bin/rpm -e --nodeps rhn-client-tools
/bin/rpm -e --nodeps yum-rhn-plugin
rpm -Uvh --force *.rpm
yum upgrade

How painless was that?!