We changed the PCs at my company a few months before the release of Windows 10, so they have been supplied with Windows 8.1 Pro (Windows 8 sticker on the tower).
At the time, we downgraded them to Win 7 Pro, but now we are planning to transition to Windows 10 Pro (clean install on the same hardware).
I have made a few tests and when doing a clean install on these computers, Windows 10 activates just fine with the embedded OEM hardware key on each PC (no volume licences / KMS involved here).
So it's working but my concern is : "Is it legal ?" I don't want to be sued by Microsoft for activated products we are not supposed to own...