This week's release of the latest Windows 10 Insider build, 10565, brought plenty of great improvements to the OS. However, one of the more interesting changes that Microsoft slipped in deals with making activation much easier for those doing a clean install of Windows 10 on a machine that previously ran Windows 7, 8, or 8.1. Here are the details from the changelog:
Device activation improvements: 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. Refer to the Insider Hub for more information on these activation improvements including requirements.
Up until now, those taking advantage of the free upgrade offer couldn't initially do a clean install because the free copy didn't include its own product key. This latest change essentially means that, when you upgrade a machine currently running Windows 7, 8, or 8.1 with a clean install of Windows 10, you'll be able to use your previous Windows 7 or 8 product key to activate the OS.
It looks like Microsoft is using some hardware ID magic on the backend to make this happen, matching the hardware setup of your older Windows activation, and subsequently its product key — to confirm that your Windows 10 install is valid. ZDNet's Mary Jo Foley has even tested this out and confirmed that it indeed works as advertised. However, keep in mind that this is only live for the current Insider preview, and won't be live for all users until the update sees a wide release.