In a recent official post in these forums this was stated...
"If the hardware configuration of your Windows 10 device changes significantly (e.g. motherboard change) Windows may require re-activation on the device. This is the same experience as prior versions of Windows (e.g. Windows 7 and Windows 8.1). The free upgrade offer will not apply to activation of Windows 10 in such scenarios where hardware changes reset Activation."
I had thought that it linked the license to your microsoft account and activated through that when you clean-installed after the upgrade, but this sounds like its hardware-ID based.. Does this mean if I upgrade my motherboard after using the free upgrade I would lose my Windows 10 license, and then have to go buy a copy of Windows 10, or would I be able to simply enter the key from my old 8.1 install during the Windows 10 install process after installing the new motherboard?
I had planned to do an upgrade to the X99 platform within the next year, but this makes me worry that I will lose my license from the upgrade as soon as I put in the new hardware.