It depends on which clients you are using.
If you have any other clients than XPe based ones, than you are fine to switch off the old WDM server, install the new one with the SAME IP and SAME name. All clients will check in just fine to the new server. You can then upgrade the hagent, if you like but that is not needed.
If the clients are XPe based and you follow the steps above all clients will check in fine as well but the clients will reboot at least three times to disable the write filter, update the system with the “new” server info and enable the WF again.
You should not have two WDM server running at the same time!