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 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?