New
#21
I can tell you what MS said to me:
I had a retail Windows 8 license which I upgraded to 10. I had always used this license to both run a VM (via VMware Fusion) and run as native boot. Whether this is valid I don't know (it is only one installed instance) but it always worked or 7 and 8. After the upgrade to 10 switching between the 2 failed irrespective which (VM or native) I upgraded as new hardware was seen and after a couple of switches it failed.
MS told me (by phone so not officially) that the upgrade license while retail channel is NOT transferrable whatever the underlying license was. They then replaced the activation I got from the upgrade with a retail Windows 10 key (for free) which lets me swap back and forth as much as I want. Clearly not all retail licenses (or perhaps I mean activations) are equal.
I think really (as long as you are being honest and have a plausible case) they will activate pretty much anything - even if it just to get you off the phone :)