Clear situation: you run a domain using Windows Server 2008 (SP1) and going to add an additional DC. But this time you plan to have a DC running Windows Server 2008 R2.
The Microsoft article describes all the preparation steps you need.
But once you start dcpromo on the new DC, you fail with an error message saying you have to run adprep /forestprep prior to add the new box as DC to the domain.
The adprep utility is resided under \support\adprep on the setup disk of Windows Server.
The clue is:
1. Copy adprep folder from setup disk of Windows Server 2008 R2 to the current DC running Windows Server 2008 (previous version).
2. Start it from there, watch for errors and expect successful finish.
3. Re-run dcpromo on the Windows Server 2008 R2 box.
Attention! Do not use adprep from Windows Server 2008 (the version of OS running on the existing DC), take the newest one (from Windows Server 2008 R2).
Update: here the schedule worked for me
1. Starting with domain running W2K8 you are going to replace the DC with W2K8 R2
2. You setup a W2K8 R2 machine and make it domain member
3. You add the AD services role to the this new machine
4. You look for setup media with W2K8R2 bits and copy the \support\adprep folder to the W2K8 OLD DC!!!
5. You open an TS to the W2K8 OLD DC and start “adprep /forestprep” – watch for messages, must finish successfully!
6. You switch to the new W2K8R2 machine and launch dcpromo promoting the new W2K8R2 box to be an additional DC in the forest.
7. Do not forget to replicate DNS on this step. Mostly the DNS service is not running on this step and will need a time to start and synchronize with the OLD DC!!!
8. After a while start “active directory sites and services”.
9. Expand until you will see the – at least two – leaf nodes: one is your OLD DC and one is the NEW ONE.
10. Create new connections for replication on the new DC and let it replicate.
a. First you may receive an error with RPC server not available
b. Wait a couple of minutes until DNS synchronizes
11. After a while comes the message about succeeded synchronization.
12. You are done, lucky you! Now the old box can be powered down…