If you have another datastore for this ESXi host with enough space to accommodate the thick disk of this VM.
Then:
1. Right click the VM > Migrate > Change Datastore
2. Advanced > Change Destination for the VMDK you want to convert to thin and Disk type as thin from drop down
3. Once VMDK is moved to a different datastore its disk type is changed to thin
4. Then you can storage vMotion this VMDK back to your original datastore.
Suhas