Is this a virtual environment or physical. If it's virtual create snapshots of both DC's, if physical use your backup software and create a full backup in case of failure(I would recommend testing the restore process to ensure the recovery) After that you need to find out the domain functional level is it running Windows Server 2000 mixed or Windows Server 2003 native, etc...In order to migrate to Win2k8 R2 as DC's you need to be running in at least Windows Server 2003 native mode. You can find this out by opening AD Domains and Trusts and right click on your domain and select "Raise domain functional level". You can raise the level to Windows Server 2003(but once you do this you can't go back). After you raise the functional level, I would just use ntdsutil to find out the "FSMO roles" that each DC holds(you can google what I have quotes, you can also use MMC to find out all the information). Next you need to update the schema master on the 2003 DC's which can be done by using adprep32.exe /forestprep from the support/adprep off the 2008R2 CD. If everything successfully updates run adprep32 /domainprep, adprep32 /domainprep /gpprep and if you plan on having read-only DC's adprep32 /rodcprep. Again if you have no bumps in the road and everything was successful. You can prep your 2008R2 member server to be a domain controller. So on the 2008 R2 run dcpromo and join it to the existing forest, make sure you make it a DNS and Global Catalog server. After your server(s) are now DC's you can safely start to transfer over roles to the new DC's. This is where you can go back to the FMSO roles and see which servers had which functions and transfer those over to the new servers. For instance if your schema and Domain were owned by test-dc1 - transfer those roles to test2k8R2-dc1. Likewise if the RID, PDC, Infrastructure were owned by test-dc2 you can transfer those roles