I am attempting to do this without powering off the Virtual Machines on each of the ESXi Hosts. Any suggestions would be greatly appreciated. Just trying to figure out all my caveats in case anything goes the wrong direction in the process.
Safest to migrate all VMs to a host in the cluster, put host into maintenance mode, unregister the host from old-vCenter, re-register host on new-vCenter, join new cluster on new-vCenter (assuming Enterprise Plus license, then...) vMotion VMs from old-vCenter to new-vCenter. If you do not have enough hosts (like 1 extra host of capacity), you'll need to power off less-favored VMs to make room.
P.S. cross-vCenter vMotion is MUCH faster if the VM is left powered on (even if you stop most useful services) because when powered on vSphere uses the vMotion network (defined with MTU=9000 on fast switches). If the VM is powered off for cross-vCenter vMotion, then vSphere will select the Management network (likely only MTU=1500 on slower switches).
That was sort of my thinking. We have Enterprise Plus Licensing for the environments. I am basically moving 2 Clusters each with 3 ESXi Hosts. So following your thought process.....vMotion everything off one Host and put in Maintenance Mode.....remove it properly from the old Cluster and then add it to the new cluster. Then basically Import the VM's from the other 2 into the one we just added till I free up another ESXi Host and basically keep repeating the process till everything is moved over....I have not used the cross-vcenter vMotion...I may look into that as well......