I had an old vSphere 6.0 environment that I didn't want to connect to my new 6.5 vCenters, so we used a shared datastore for migrations. Basically I had a migration datastore that was accessible and mounted to both vCenters, then I'd s-vMotion the vm being migrated to the datastore, power down and remove the vm from the inventory of the source vCenter and add to the inventory on the destination vCenter. Make any config changes necessary for network etc and power on.
I'm sure there a several valid methods depending on your needs, but this one worked for us.