After converting disk using vmkfstools command. Remove thick disk from VM's edit settings and add it as existing disk. Check vmdk it should be thin provision now. I remove thick vmdk from datastore.
After doing this I notice storage space is still same. I was expecting to free some space than I deleted flat file related to thick disk. Now I have free space.
These things are tested on VMDK which is running without snpashot. If your VM has snapshot than it can create problem.