SUCCESS! I set up a VM from my Win 10 Pro Bootcamp, which was activated by upgrade from Win 7 ultimate retail DVD. I went to activation in the VM and saw the "couldn't activate" red writing, with option to buy from Microsoft Store. I went along the buy from Store route to see the cost (£189). This involved signing in to my Microsoft account. At that point there was also a clickable link saying "I already have a valid license" So I clicked that and was asked for my product key. I entered the product key from the Windows 7 DVD ultimate retail box, more in hope than expectation. Then it reported "this PC is now activated using the key in your Microsoft account", or similar. Booted back to Bootcamp ...still activated Booted back to VM sill activated. Office 365 is also activated with no special steps.