Who wants to trash a valid working version of 7 or 8 rather than beta test a fresh install of Windows 10. This would be great if the free license was for those who install and beta test a fresh version of Windows 10 on a PC without having to upgrade an existing copy first.
Well, lucky you - because that is exactly how it is. You download the (free) technical preview build ISO http://windows.microsoft.com/e..., install it, and then later on it will be upgraded to a (free) fully licensed copy. Only requirement is that you have to sign up for a Microsoft account (just create some new email address for that).
Or, in easier words, the full version of Windows 10 is free for everybody who participates in the technical preview.