Starting in Windows 10 Insider Preview build 10565 and TH2 Build 10568:
Device activation improvements: Microsoft has received a lot of feedback from Insiders on making it easier to activate Windows 10
on devices that take advantage of the free upgrade offer to genuine Windows by using existing Windows 7, Windows 8 or Windows 8.1 product keys. If you install this build of the Windows 10 Insider Preview on a PC and it doesn’t automatically activate, you can enter the product key from a qualifying Windows 7, Windows 8 or Windows 8.1 used to activate the prior Windows version on the same device to activate Windows 10 by going to Settings > Update & security > Activation
and selecting Change Product Key.
If you haven’t upgraded to Windows 10 yet and perform a clean installation, you’ll need to enter a qualifying product key for Windows 7, Windows 8.1 or Windows 10 or buy a license in order to activate Windows 10.