if you have only 2 hosts... and few vm... then just intall the new vcenter and then add the esxi hosts..to the new vcenter, in old vcenter just remove the old esxi hosts.
After adding to the new vcenter, just evacuate the vms in the esxi host, and eiter upgrade or do a fresh install.
once the esxi latest, you need to format the VMFS to 3 to 5, then only you will get the all the latest VAAI features.. and other optimization in the VMFS5.
adding and disconnecting a host from vcenter wont do any harm