Adding Server 2008 domain controller

[brunei2730]

My primary domain controller is Server 2003. Before I started here they attempted to add a Server 2012 backup domain controller, but ran into errors and didn't fully fix them. This has left me with a backup domain controller that doesn't respond when the primary one goes down. The schema version is being listed as 56, which makes sense.

I now want to add in a second backup domain controller, but this time Server 2008. Do I need to run the adprep command on the 2003 server or can I just go right into setting up the roles on the 2008 Server since the schema is above what I need?

Thanks.