there was an old thread about it at the time of 4.0/4.1. 

What it worked and I verified still works in 4.5 is:

VM is powered on or off (with pros and cons for both scenarios in case of production machine)

create a snapshot of the VM 

select the snapshot line under snapshots and select clone button

in resource allocation section of the new vm screen change allocation policy for the desired disk/s from thin provision to preallocated

Then you can decommission old VM and use the cloned one

HIH,

Gianluca