When looking to upgrade your Active Directory system and Domain Controllers from a Windows Server 2003 to 2008, you will need to go over the Checklist provided by Microsoft. One of the first steps in the checklist consist of preparing your environment to properly work with 2008. The following command will prepare your current AD and DC.
Load the Windows Server 2008 installation cd & navigate to adprep folder via comand promp: D:\sources\adprep
Now run the following command, which will prepare the forest schema:
We will also need to run another command:
adprep /domainprep /gpprep