You want to upgrade your existing Windows 2000 Active Directory domain controllers to Windows Server 2003. Before doing this, you must run the ADPrep tool, which extends the schema and adds several objects in Active Directory that are necessary for new features and enhancements.
First, run the following command on the Schema FSMO with the credentials of an account that is in both the Enterprise Admins and Schema Admins groups:
> adprep /forestprep
After the updates from /forestprep have replicated throughout the forest (see Recipe 2.11), run the following command on the Infrastructure FSMO in each domain with the credentials of an account in the Domain Admins group:
> adprep /domainprep
If the updates from /forestprep have not replicated to at least the Infrastructure FSMO servers in each domain, an error will be returned when running /domainprep. To debug any problems you encounter, see the ADPrep log files located at %SystemRoot%\System32\Debug\Adprep\Logs.
The adprep command prepares a Windows 2000 forest and domains for Windows Server 2003. Both /forestprep and /domainprep must be run before you can upgrade any domain controllers to Windows Server 2003 or install new Windows Server 2003 domain controllers.
The adprep command serves a similar function to the Exchange 2000 setup /forestprep and /domainprep commands, which prepare an Active Directory forest and domains for Exchange 2000. The adprep /forestprep command extends the schema and modifies some default security descriptors, which is why it must run on the Schema FSMO and under the credentials of someone in both the Schema Admins and Enterprise Admins groups. In addition, the adprep /forestprep and /domainprep commands add new objects throughout the forest, many of which are necessary for new features supported in Windows Server 2003 Active Directory.
If you've installed Exchange 2000 or Services For Unix 2.0 in your forest prior to running adprep, there are schema conflicts with the adprep schema extensions that you'll need to fix first. MS KB 325379 and 314649 have a detailed list of compatibility issues and resolutions.
Recipe 2.11 for determining if ADPrep has completed, Chapter 14 of Active Directory, Second Edition for upgrading to Windows Server 2003, MS KB 331161 (List of Fixes to Use on Windows 2000 Domain Controllers Before You Run the Adprep/Forestprep Command), MS KB 314649 (Windows Server 2003 ADPREP Command Causes Mangled Attributes in Windows 2000 Forests That Contain Exchange 2000 Servers), and MS KB 325379 (Upgrade Windows 2000 Domain Controllers to Windows Server 2003)