It does only work if you don't use Fast Provisioning (Linked Clones).
The workflow "Change Hard disk capacity" doesn't work in 5.1.2.
It creates the error:
The hard drives of the virtual machine "Lin" may not be modified, once created.; majorErrorCode=400; minorErrorCode=BAD_REQUEST; vendorSpecificErrorCode=null; (Workflow:Change hard disk capacity / Resize Disk # (item0)#15)