[CentOS] Migrate RHEL3 to Cent3

I have several RHEL3 WS machines that were installed here running as  
servers and their RHN subscriptions were never kept up. Now that I  
have become responsible for them I would like to move them to CentOS  
as I can keep them updated as needed.

Some of them I can simply migrate to new machines, but a few I can  
not. Is it possible to simply upgrade a RHEL3 to the latest CentOS 3?

If possible, once I have it to the latest Cent 3 can I then further  
upgrade the machine to the latest CentOS 4?

-Thank you