Where do you store your VMs are they stored in local datastore or central storage SAN?
You should add all your ESXi hosts to the vCenter Cluster so as to enable vMotion and migration of your VMs from ESX to ESXi hosts.
Once the VMs are moved out of ESX host format it, install ESXi and add to vCenter cluster, after having all ESX server formatted and installed with ESXi you can then configure your vCenter for HA,DRS,vMotion etc.
Nikhil