Update: Microsoft silently updated their original blog post, reaffirming that a genuine Windows 7 and Windows 8.1 license is required for a legitimate activated copy of Windows 10. Original story follows.
Microsoft published a blog post last night which spoke about a number of things which are changing for Windows Insiders on the heels of the Windows 10 RTM, which is set to launch on July 29th. Within the blog post, the company explains how upgrading to the Windows 10 RTM will work for Insiders, being that it'll arrive much like any normal Windows 10 build via Windows Update. They also talk about activation, and how Insiders will remain activated even once RTM hits.
Microsoft has claimed in the past that users upgrading from Windows 7 and Windows 8.1 will receive Windows 10 for free, however that was never the case for Windows Insiders who installed clean. Microsoft consistently said that you would need an existing Windows 7 or Windows 8.1 license to get activated on Windows 10, but now that seems to have changed.
Although the blog post doesn't specifically state that Windows Insiders get Windows 10 for free, it does explain the process:
As long as you are running an Insider Preview build and connected with the MSA you used to register, you will receive the Windows 10 final release build and remain activated. Once you have successfully installed this build and activated, you will also be able to clean install on that PC from final media if you want to start over fresh.
Gabe Aul also clarifies the process on Twitter:
Also take note, Microsoft specifically mentions the requirement of having a Microsoft Account registered as an Insider to remain activated. So those who want Windows 10 for free without wanting to be part of the Insider Program or have a Microsoft Account synced to their PC will have to miss out, or upgrade legitimately from Windows 7 and Windows 8.1.
So there you have it. It definitely looks like Microsoft is giving Windows 10 away for free to Windows Insiders, regardless of if you had a Windows 7 or Windows 8.1 install prior to installing Windows 10. That's definitely a nice treat and a great way to say thank you to Insiders for helping test Windows 10, don't you think?