While migrating from one vCenter to another the VMs can either be copied or moved, if copied, that would create new identities for each VM. This means the VM would be considered as new VM, so no historical metrics will be retained on the new vCenter in Foglight.
That is the reason why, when asked by vSphere client if the VMs are being imported or copied, one should choose "I moved it".
For more information check the following:
It has been tested on Foglight for Virtualization Enterprise 7.2 and another 8.3 instance.
Note that both bios.uuid and vc.uuid (our vmId) are the same after the move. If using a method to move VMs that would replace bios.uuid (for FVE 8.2 or earlier) or vc.uuid (for 8.3 or newer) then it would see the moved VM as a new VM.
After migrated, stop data collection on the old vCenter and create the new vCenter agent, after migrating ESX host and all corresponding objects from one vCenter to another, those objects migrated, the historical information will remain on the system with the new vCenter migrated.