in principal it should be possible to update from RHEL to CentOS, because both distributions has the same source packages. But from my point, i think i'll never try this, because it can result in a completely unusable system.
Best way is reinstallation to prevent from an unbootable system.
You can download CentOS from different mirrors, also as CD images. For example at ftp://ftp.tu-chemnitz.de/.SAN0/pub/l...5.3/isos/i386/
There is also a small netinstall.iso available for CentOS, so it is not required to download complete CD's.