In the past couple of months, I have Built and upgraded 2 PC's, Both are windows 10 pro and were activated with Windows 7 pro OEM keys... I bought for £10.
I think MS is just trying to get rid of all Windows 7/8/8.1 pc's out there, so are still ok with trading Old Win 7 keys for new.
My own Win 8.1 retail key was upgraded to Win 10 pro and they gave me a new retail version
