Under the settings of some of my virtual machines, the provisioned size for the disks are grayed out so I can't make them bigger. I have read other articles pertaining to this and:
There are no snapshots
The machine is powered down
Those are the only two solutions I can find. I have seen where it is suggested to use the converter tool, but I'd rather not use that. I used that before and it broke the licensing for the application that runs on the virtual machine and I had to get new keys.