[CentOS] CentOS to RedHat and vice versa

Has anyone ever tried to mass replace installed RedHat RPMS with their
equivalent CentOS versions or vice versa?

I was thinking of generating a list of all packages and then running
RPM or yum with a 'replace' option.

The reason for doing this:
Years ago I was tasked with building a RHEL4 system. Budgeting, time
constraints, the lunar cycle, etc.., prevented the purchase of an
equivalent development environment. So CentOS was used for the
development/test system. It sufficed for several years.  Now I need to
make them as close to identical as possible which means registering
the development system in RHN and making it "standard".

I've seen posts that RHEL -> CentOS is at least possible. That is,
take a RHEL system and get it to update via YUM and CentOS
repositories.  I have not seen the reverse, however.