After replacing or upgrading some storage on your VMware ESXi server it could be useful or mandatory to move some VM to a new datastore.

The process is not too complex but it requires some explanation:

• Power off VM
• (Optional) Consolidate snapshots if needed.
• Remove VM from vCenter inventory
  • Right click VM and click “Remove from Inventory" 
• Enable SSH on the ESXi machine
  • In the vSphere client go to: Configuration -> Security profile -> Properties (next to Services) -> SSH (in the list) -> Options -> Start
• Log in via SSH as root
• Prepare a directory on the destination datastore
  • mkdir "/vmfs/volumes/destination_datastore/Some VM"
• Clone the .vmdk files using thin provisioning
  • vmkfstools -i "/vmfs/volumes/source_datastore/Some VM/Some VM.vmdk" -d thin "/vmfs/volumes/destination_datastore/Some VM/Some VM.vmdk"
• Copy any remaining files (avoiding overwriting the .vmdk files)
  • find "/vmfs/volumes/source_datastore/Some VM" -maxdepth 1 -type f -print0 | grep -v ".vmdk" | while read file; do cp "$file" "/vmfs/volumes/destination_datastore/Some VM"; done
• If you did not consolidate snapshots in step 2, there may be snapshot .vmdk delta files, we also need to copy these (this may take some time):
  • find "/vmfs/volumes/source_datastore/Some VM" -maxdepth 1 -type f -print0 | grep [0123456789][0123456789][0123456789][0123456789][0123456789][0123456789] | grep ".vmdk" | while read file; do cp "$file" "/vmfs/volumes/destination_datastore/Some VM"; done
• Once done cloning and copying all necessary files, add the VM from the new datastore back to inventory
  • In the vSphere client go to: Configuration->Storage->Data Browser, right click the destination datastore which you moved your VM to and click "Browse datastore”. 
• Browse to your VM and right click the .vmx file, then click “Add to inventory" 
• Boot up the VM to see if it works, when asked whether you copied or moved it, just answer that you copied it. (I’m not sure what this means, but I think it has to do at least with the MAC address of the vNIC being changed.) 
• If the VM boots up fine, you can remove the VM from the old datastore.
• rm -rf "/vmfs/volumes/source_datastore/Some VM"

Source: http://serverfault.com/questions/372526/move-vmware-esxi-vm-to-new-datastore-preserve-thin-provisioning