[CentOS] migration from redhat AS3/4

We have 30 or so boxes that are currently running either redhat as3 or as4 and would like to migrate them to Centos 4.2.

We have a local centos 4.2 repository using yam. I have in the past updated es3 to as3 by removing the redhat-release rpm and installing the other redhat rpm. I realize there are virtually no differences in es and as3, except for access to additional packages. In any case, on the AS4 boxes would we be able to do the same thing? Remove the redhat-release rpm and install the centos-release file then switch the yum repository and update? Or are we talking a reinstall?

I doubt that it will work from AS3 however, but I thought I'd throw that in as well.

Thanks in advance,

Mark