I'm trying to get a better understanding about how Active Directory handles Schema updates, specifically how safe the procedure actually is given how critical AD is and given the range of situations where updates are required. Exchange 2007, OCS, SCOM all require schema changes for example, it's not just something that happens when you are considering a major shift from (say) a Windows 2003 to a Windows 2008 infrastructure.
What I'm looking for is advice on the best backout plan for schema changes, just in case it actually does go wrong. Would it be acceptable to take one DC offline during the update, for example, and use that to roll back the entire environment if the schema update failed? Are there any problems with reactivating a DC that was offline during a schema update?