From the information scattered around the Internet, it seems that running Windows as a host OS and also installing it as a guest OS somewhere using the same key is okay so long you don't use both at the same time.
I'm looking to migrate to Linux and I'd like my host OS to be say Ubuntu for starters and my guest OS to be Windows. My computer came with Windows 8.1 Home OEM licence and I clean installed Windows 10 Home when it came by which I got the free upgrade and the Windows 10 install was activated just by the virtue of running on the same hardware, I didn't have to enter my key again and a generic key is shown in Windows 10.
Given that I upgraded the OS and that the host OS I aim for will not be Windows, can I use Windows 10 Home with my original key and use it in the VM? Or will I need to install Windows 8.1, activate it with the original key, then upgrade it to Windows 10 and get a free update for the guest OS install again? Do I install Windows 10 straight away and expect it to be automagically activated like my host OS install was? (I expect not, since the HW ID will not be the same.) Can I use Windows with a OEM licence which is for older version as a guest OS when not on Windows host OS at all?