There are various scenarios in which you may need to unregister a cluster from PC, and it is important to do it correctly. 

Whether you are decommissioning a Prism Element (PE) cluster which was registered to Prism Central (PC), or you already have decommissioned a cluster but it is still linked to a PC, or you have a cluster that is registered with one PC instance but would like to re-register it with a different PC instance for the benefit of localized management or to configure availability groups using Leap. All 

What does "correctly" look like?

Done properly, unregistration of a cluster from PC involves a remove-from-multicluster step followed by clean-up of associated metadata. This metadata clean-up must be allowed to complete prior to attempting to re-register the cluster to a PC, otherwise, registration could be blocked.

How to do it?

There is no GUI method to unregister a cluster from Prism Central, so the process requires SSH access to the PC VM as well as to a CVM of the cluster. Please use "nutanix" user to authenticate with both the PC and the CVM.

Depending on the scenario at hand the process differs slightly.

1. If the PE cluster still exists and can communicate with Prism Central, you’ll need to get the UUID of the cluster from the CVM and then use that to trigger de-registration from PC command line.

2. If both the PE and PC cluster still exist but cannot communicate because the PE cluster was relocated or changed to a different network, you will need to complete the un-register and clean-up steps on both PE and PC.

If the PE cluster or the PC VM(s) was destroyed without completing these steps, you should complete unregister and clean-up steps on whichever the system remains (the cluster or the PC), following the same method as in scenario 2.

The full steps to unregister a PE cluster from PC are covered in the article KB-4944 Unregister from Prism Central and cleanup | Force unregister PE from PC after PC is deleted.