Can you set up a sacrificial test machine? I had thought that something about this was in the CentOS faq, but it isn't--nor did a quick search of the forums find an answer.
A bit more googling around indicated it's possible, and in theory, it seems as if it should work. There have to be some guides around, I just haven't found any.
Ah, on the CentOS wiki, I should have gone there first.
http://wiki.centos.org/HowTos/MigrationGuide
Look for
Migration from RHEL5 to CentOS5