Windows 10 Insider Preview Build 10565 released yesterday with numerous functional improvements, but also a key change in the activation scheme: Windows 7, 8, and 8.1 product keys can be used to activate Windows 10. Previously, you could install these operating systems as normal and then upgrade to Windows 10 for free; Microsoft is mercifully making that step unnecessary by allowing you to simply enter in the product key of the previous OS to validate your new Windows 10 install.
There is a stipulation: if you're going through the upgrade process instead or clean installing, the key you use with the PC you've installed Windows 10 on must have been used to activate the previous installed version of Windows. So for example, if you installed Windows 7 on your PC and then want to upgrade to Windows 10, you can do so, but you can't install Windows 7 on your laptop and then use that key to upgrade to Windows 10 on your desktop and then activate it (you'd have to have installed it on your desktop first at some point).
To start the process, go to Settings > Update & security > Activation, and then select Change Product Key if upgrading, and if clean installing, simply use the key during setup.
Presumably, this change will go live for everyone and not just Insider members soon.