[CentOS] How to convert RHEL 4 system to CentOS

Hello Everyone,

I have a RHEL4 system that I would prefer to leave on the current 
hardware and not have to rebuild if possible. Can some tell me if I can 
take my current distro of RHEL and move it to CentOS? What will I need 
to do, etc and how risky is it?

Thanks,

James Marcinek