Question : Use Windows Server with SBS 2003

Can I use a Windows Server 2003 or even 2008 R2 as an additional domain controller in an SBS2003 domain?

Answer : Use Windows Server with SBS 2003

You need to run adprep32 from the 2008 DVD on the SBS server.

See the step here in my article: http://www.experts-exchange.com/articles/Software/Server_Software/Email_Servers/Exchange/Migrate-Small-Business-Server-2003-to-Exchange-2010-and-Windows-2008-R2.html