I am currently running Vcenter 6.7 and ESXi 6.0 with VMFS5 in our infrastructure. And I often have to move VM disks from a DataStore to another in order to free up space.
When using the VCenter MOVE operation on a select VM Subfolder in the Datastore File explorer and moving it to another datastore, the VMDK file which was thin provisioned and used 35GB was converted to thickly provisioned disk weighting 100GB after the move to the new Datastore.
When using the VCenter MIGRATE operation on a selected VM Machine inside the linked ESXi host (only one host connected to VCenter per DataCenter(3)) we are able to select the migrate storage only, see pic: https://imgur.com/a/zdXP99T and obtain the VMDK migrated and still thin provisioned. BUT we are only able to perform this MIGRATE operation on VMs that are inside the connected Vcenter's Esxi host and not other VMs folder hosted on different ESXI host.
Here is little drawing explaining this. https://i.imgur.com/hG5cqoq.png
I've read about the Unmap command but I am not sure if it applies to actual VMDK or whole Datastores.