... OK, so I successfully converted all my VMs to SATA, except for the Windows 7 Pro one, which bluescreens when booting (after the animated Windows logo), thus triggering the troubleshooting tool (or whatever it's called...), which sadly solves nothing: same blue screen at the next reboot.
So, it looks like an existing Windows 7 VM cannot be converted from SCSI to SATA (both HD and CD/DVD)...?
Well, anyway, I can always recreate the VM ex novo, when I have some time - no problem...