The latest Threshold 2 preview release, build 10565, contains a welcome change.
This build eliminates activation hazard in Windows 10. Now you can use a corresponding edition of Windows 7, Windows 8, or Windows 8.1 product key on that device where you are clean installing Windows 10.
The product key changes coming in Threshold 2 offer a third alternative. Here’s the official announcement:
We have 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 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 do a clean install of Windows 10 by booting off the media, you can also enter the product key from prior Windows versions on qualifying devices during setup