Due to some issues with Windows 10 activation, Microsoft has added support for older Windows product keys in the latest Windows 10 insider beta build. The change will allow users to activate the OS with a serial key of Windows 7, Windows 8 or Windows 8.1. Users will be able to input these keys directly during a boot media install of Windows 10 but it's not clear if you can now do a clean install of Windows 10 without doing an in-place upgrade at least once.
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.
The new Windows 10 insider build also include a bunch of other tweaks and changes, like better Skype integration, colored window title bars, and some Edge and Cortana tweaks.