Each VMFS datastore has a unique identifier (UUID).When you attempt to mount a device that contains a VMFS datastore copy/storage snapshot, you have an option of mounting with its original UUID or changing the UUID to re-signature the datastore.
Besides that VMware® vSphere® VMFS-5 Upgrade Considerations might be useful to take a look.
when you have that terrible old stuff like 4.0 you *really* should migrate to a new storage and if it's temporary something like NFS on a cheap hp microserver, after that you can cleanly format your storage with the newest VMFS and then migrate the running guests back
Thank you for your reply.
Doesn't the migration to ESXi 5.5 automatically upgrade VMFS to 5?
Since ESXi 5.5 still supports VMFS3, the upgrade to from VMFS-3 to VMFS-5 for the ESXi 5.x can be done either via command-line or via vSphere Client.
In ESXi 6.7 because it no longer supports VMFS3 datastores. The ESXi host automatically upgrades VMFS3 to VMFS5 at the first boot after an upgrade to ESXi 6.7 or later, when the host mounts all discovered VMFS3 datastores.