You have some choices, either create a disk image and write it back to a new SCSI virtual hard disk or you'd have to change information in the Disk DescriptorFile of the virtual hard disk. With "twoGbMaxExtentSparse" type disks then is easy as the first Extent is a text file not a binary file link the remaining Extents and with "monolithicSparse" the Disk DescriptorFile is embedded in the binary file and would need to be extracted, modified and injected back. In your OP you said they were "single file hard drives" do I'm assuming they're "monolithicSparse" so if you're not comfortable using Terminal and dd then creating a disk image and writing it back to new SCSI virtual hard disks is what I'd recommend. If they are "monolithicSparse" and you want to pursue using Terminal and dd then you need to first make a proper backup of the Virtual Machine Package as making mistakes with dd can destroy the virtual hard disk.
So if they are "monolithicSparse" and you want directions using dd to extract/inject the Disk DescriptorFile in order to change the adapter type and geometry of the virtual hard disk.