Edit: No I'm wrong. I somehow completely missed that one question in the faq when I was reading it earlier that actually answers this exact question;
Will it? Will it really work? Time will tell but knowing Microsoft I've got a feeling come the end of the free upgrade period that it's going to be a bit more complicated than their activation servers simply "knowing" that your old windows 7/8 key was already upgraded and allowing windows 10 to activate.To clarify, in order to transfer your copy of Windows 10 to a new PC, assuming you have a retail license of Windows 7/8.x, you must first install Windows 7/8.x on the new PC, then perform a new upgrade on that PC. If this happens after the 1 year upgrade period is over, the Activation servers will know that your old license has already been upgraded and re-activate the new upgrade.