A simple script to convert a rhel box to a centos box
This has not been extensively tested. I created 2 rhel boxes (5,6) and ran this script multiple times
and it worked for me. It MAY not work for you (who knows what crazy setup you have).
PLEASE test this before using it on ANY production boxes. In fact, I cannot endorse it, use at your own risk!
PLEASE READ THE FAQ BELOW!!!!

Only RHEL 5 and 6 are supported, not minor releases
pass it the version you are changing to
e.x convert.sh 5 would convert a rhel 5 box to a centos 5 box
change the mirror to a mirror that has all minor releases if you want minor versions to work
Change the MIRROR variable in the script to a different mirror if you do not like mirror.centos.org

FAQ:
why not language X instead of bash:
* It was super quick to write this, and why complicate things. 

Why can I not go from rhel 6.1 to centos 6.1? Why does it force me to Centos  (latest)?
* By default, when there is a new minor release (say CentOS 6.3 to 6.4), the packages for 6.3
are essentially deleted in the mirror (no more updages) and you are automatically upgraded to the 6.4 (in this example)
If the current latest version is 6.4 and you are running RHEL 6.1, to go to Centos 6.1 you would need a mirror that has
all of 6.1 packages mirrored. Otherwise if you use this script and you are running RHEL 6.1 and you do convert.sh 6, you WILL
BE UPGRADED TO THE LATEST VERSION OF CENTOS.