"Migrate from Previous Versions to Windows Server 2012 R2 Essentials or Windows Server Essentials Experience".   What I need to know is where in the process does the old server stop functioning as a server for my network.   What I mean by this is when in the migration process (when I add the new server as a replicate DC?  not until I demote the old server?) will any programs/databases on the old server no longer be available to client computers?  

Thanks for any insight.