So without registering old hosts to the ESX hosts, i could just copy VMs files manually over the network...Is that what you suggest?
The benefit of registering old hosts is that i could then use SVmotion and Vmotion to migrate VMs and eventually avoid some downtime.
With shared storage and the appropriate licenses in place it's certainly a lot easier to add the old hosts to the new vCenter Server and use VMotion (in case the hosts are compatible). Once all VMs run on the new hosts and the old hosts are gone, you can either upgrade the existing datastores from VMFS-3 to VMFS-5 or - if you prefer - SVMotion the VMs to the newly created datastore(s).
Not adding the old hosts to the new vCenter Server would require a short downtime, i.e. power off the VM's, unregister from the old inventory, register on the new inventory, power on the VMs.
It's basically your decision whether to upgrade the old hosts to be able to add them to the new vCenter Server instance or accept a short downtime.
André