Windows 10 1709 Upgrade Removed 'Saved Critical' and 'Off Critical' Hyper-V machines

Following some investigation, it appears that the following has happened:

• All VMs with configuration version lower than 7.0 have been removed, even though 1709 should support down to 5.0.
• The files in C:\ProgramData\Microsoft\Windows\Hyper-V\Virtual Machines appear to reference the *missing* VMs. As such, references to the original VM config file have been rehomed to point at my default VM location (C:\Virtualisation in my case). Originally I didnt dig too far and as the number of files in this folder matched the number of VMs I had listed, I foolishly thought that this was the list of current VMs...
• Copying the files for a missing VM within the existing tree into the location referenced by the missing VM file in C:\ProgramData\Microsoft\Windows\Hyper-V\Virtual Machines and then restarting the Hyper-V Virtual Machine Management service causes the VM to reappear in Hyper-V Manager.

My next test is to move the copied VM to the location I want it in on an external disk and then upgrade the configuration file to the latest version and ensure that all behaves as expected.

Note: I attempted to re-import the missing VMs from original locations and got the following error: