Install an additional site (additional SEP 11 manager) in your network and configure as replication partner the first manager. After you configure this, durring the installation of your second manager, a replication will occur. As soon as replication finish, apply to your clients new Management Server List (a new MSL will be created automatically with your second installation and you should use it), so they will know that a new server exists and they should use it as primary management server. After you done this, you can uninstall your first instance of SEP manager.