OSX Mavericks and OSX Lion Login Server
Updating Server.app is not really necessary. OS X Mavericks clients are backward compatible with OS X Lion Server, in the same way Mavericks Server is backward compatible with Lion clients. It is only required to update Server.app if you want the new features available in the updated OS X Server, or if you wish to upgrade the host OS on the server (i.e. Lion Server.app is not compatible with OS X Mavericks).