We were out at a customer's site yesterday installing a new Windows 2003 R2 Standard Edition server, and when trying to run DCPROMO to turn the new server into a domain controller we kept getting the message "The version of the Active Directory schema of the source forest is not compatible with the version of Active Directory on this computer."

Our existing server was a Windows 2003 Standard Edition server running Service Pack 2, so I couldn't understand why I'd need to run ADPREP, but I figured the system knows better than I.  I run ADPREP /FORESTPREP from DISK 1 of the Windows 2003 R2 CDs and it tells me everything is fine, that no upgrades to the AD Schema are necessary.  Well after some further investigation, I find there is a NEW version of ADPREP on DISK 2 of the Windows 2003 Server R2 CDs in the \CMPNENTS\R2\ADPREP folder.  When i ran this newly found version, the AD Schema was upgraded and the next attempt at running DCPROMO was successful.

I wish I understood why Microsoft would have hidden the new ADPREP on the second CD a couple of sub-folders deep instead of just replacing the ADPREP on the first CD, but oh well....Problem solved.