Hardware Upgrade on Domain Controller Windows 2003

Hi all,
im goin to upgrade the domain Controller (primary) which also holds the DNS.
this server has 5 sites connected to it.
each site has its domain controller which has been installed as additional domain controller to this one.
Recently i upgraded one of the site’s server and the OS came up with the new machine was server 2003 std R2 to co-exists in 2003 server std network , i had updated the schema at primary Domain controller, the operation master.

Now im upgrading the hardware of Primary Domain controller later on this week, i want to know what exactly the steps to be followed.
and that R2 additional domain controller will cause any problem later with newly created 2003 AD schema???