since old ip is not reachable you won't able to unregister from the prism , you need to forcefully remove from the CVM , (https://portal.nutanix.com/kb/4492)
Note: You might see data related to the old the vCenter Server in the vpxa.cfgfile. In this case, do one of the following:
For more information, see the VMware KB article 1001493.
What if the vCenter was deleted and it is still registered on Prism? I don't think method 1 or 2 works. Is there another way to remove it manually? I couldn't find anything online so I'll probably re-foundation my lab environment.
I had the same problem, I can solve with the next procedure:
nutanix@cvm$ ncli ms unregister ip-address=vcenter_ip admin-username=username admin-password=password port=port_number
Replace vcenter_ip with the IP address of vCenter Server.
Replace username and password with the user name and password of vCenter Server or a random one if you receive the "the vCenter Server does not exist anymore" alert.
Replace port_number with the port number of the vCenter Server.
nutanix@cvm$ ncli ms register ip-address=vcenter_ip admin-username=username admin-password=password port=port_number
nutanix@cvm$ ncli ms list-management-server-info
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