[CentOS] migrating from rhel 3 to centos 3

Greetings - I have a rhel 3 server that I want to migrate to centos.  
The FAQ:
http://www.centos.org/modules/smartfaq/faq.php?faqid=19

pretty much tells me what I need to do but what is going to happen when 
I run yum update?  Will it replace every RPM on my system with its 
centos counterpart or will it only upgrade RPMS as new versions are 
released? This is a production system and I'm trying to figure out what 
apps will potentially be affected by the migration.

Also, should I uninstall the up2date package when I'm done?

Thanks!
Mike