Hi David,
you can disconnect you hosts (with running VMs on it) from your old vCenter and add it to your new one.
Please be aware of some other limitations:
-ESX 4.x hosts can´t see & access VMFS5 datastores
-If the hosts in the old & new vCenter instance have different CPU types you may need to check the EVC cluster settings (if not already done).
Regards,
Mario