My production system has two Windows Server 2003 R2 servers acting as domain controllers. Can I add a Windows Server 2008 DC to the existing domain without changing the schema? This is a similar question to http://serverfault.com/questions/118774/can-a-windows-2008-r2-server-join-a-2003-domain but in that case, the 2008 server being added was not a domain controller.
No. The process for adding a 2008 DC requires schema updates. Member servers are not a problem, but a Domain Controller is another thing entirely.
That said, generally speaking schema updates like those that come with Server 2008 are harmless to lower versions. Do you have a specific problem with the schema updates that come with 2008?