Before converting the VMDK file power off the VM and remove from inventory.
ssh to the esx server that you store the VM server that needs conversion
Go to /vmfs/volumes/YourStore/YourVMlocation
and run the below command to convert the HDD
vmkfstools -i ThickHDD.vmdk ThinHDD.vmdk -d ‘thin’ -a lsilogic
When is done add the VM again on the ESX server and remove the Thickhdd.vmdk and add the new Thinhdd.vmdk
Power on the VM.