I read somewhere in the MDL forum that if I upgrade my Windows 8.1 to Windows 10 my activation will remain and that after that I can do a clean install. I upgraded my Windows 8.1 to Windows 10 and my activation did remain. When I perform a clean install of Windows 10 should I format my C:\ drive? Side note: I cannot try this in a virtual machine, because my computer does not have enough resources, so that's why I decided post my question here.
It should auto-reactivate on clean install, yes. Just make sure you re-install the same version you just upgraded to.