Although converting the VM will create a new VM - i.e. leaving the original files untouched - remember that the clone will have a new UUID and MAC address by default and may need to adjust settings. That said, I don't really see a reason why you couldn't just delete the snapshots from the Snapshot Manager. The virtual disks are thick provisioned and you have enough free disk space. Deleting the snapshots will certainly take some time (at least some hours) and put some load on the system, so you may consider to do this off business hours or on a weekend. At least deleting the snapshots can be done with the VM up and running.
André