I recently installed a new Server 2012 machine on a small network to replace our aging Server 2003 DC. I have completed the migration of Active Directory, DNS, FSMO roles, made the 2012 machine the schema master, etc. Everything has been running fine for a few days. The only roles still left are file and print server. Everywhere I have looked it is mentioned to remove the 2003 server from Global Catalog Server, demote it, and shut it down. The hardware is still OK and the domain and forest functional levels are at 2003. Is it OK to keep this 2003 server online as a failover DC to create some redundancy? Would I need to do anything specific to ensure they get along?