I'm not sure about cloning the database to be honest. Whenever I have upgraded /migrated between versions its usually quick and easier to recreate the settings manually unless you have a crazy sized vsphere farm. Other things you'll need to consider is do you have any services that tie into vcenter such as veeam, vmware view, xendesktop or host based av as they will need reconfiguring too.
I would look around for a white paper on cloning the database just to make sure it'll upgrade correctly on a new vcenter box.