It is not possible to convert disk files from thick to thin, in-place. Instead, cloning the disk (or machine) allows you to change the provisioning type and thus enabling to have a thin disk. After the clone is complete, delete the original and you're good to go.
You can either clone a complete machine, or just clone a disk using the CLI or service console:
vmkfstools -i <srcDisk.vmdk> -d thin <dstDisk.vmdk>