Sounds like a good plan 
If I disconnect the hosts that should turn off the vms right?
No, Disconnecting the hosts from vCenter will not turn them off. They will run fine but they won't be managed centrally/. Once you add the hosts back to new vCenter they all can be managed centrally.
I would suggest you to disconnect and then remove the hosts from the old vCenter so that the vCenter agent will be uninstalled successfully.
You have also done your homework on interoperability which is good. I do not see any other issues here. Hope you are not running vCenter(5.0U3) with express edition. Be aware that it supports smaller inventory. Just be cautious about that.