Cloning one server to another is quite easy. Go to Tools in the top menu bar, select the Server Clone in the Tools. Select your Origin server and your Destination server, and click start task.
Each site on the new server will have GridPane as its system user. If you create matching system users on the new server prior to initiating the clone, your new sites will retain their old system users.
Once the process completes, check your sites, and flip DNS to the new server. As a recommendation, we would suggest keeping your old servers for two weeks to ensure that everything is good before decommissioning them.