When using Local Storage or Iscsi SAN the storage is managed by the ESX server that formats the given VMDK size with VMFS.
The same is with VMware Server.
When using NFS the storage is managed by Netapp and there for allows Thin Provisioning.
Regarding your problem, I would try installing the converter as an agent inside the VM and then Export it to the ESX.
In that way the Converter will export the data into new VMDK and not Copy it.
If not try using Image software , create new VM and the deploy the image into it.
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One thing I have done before is to create a new VM on the NFS datastore and then use something like Ghost to image the old VM's data into the new VMDK on the NFS datastore.
If you are comfortable on the command line, you can try something else... use vmkfstools to clone the old disk into a new, thin-provisioned disk:
vmkfstools -i /vmfs/volumes/LOCAL-DATASTORE/myOld.vmdk -d thin /vmfs/volumes/NFS-DATASTORE/myNew.vmdk