With preview versions of preview Windows releases, such an upgrade wasn't possible and required a new installation of the operating system.
At first there was some misunderstanding as his response seemed to imply that upgrading from Winows 10 Preview to RTM would be free, but he later responded that testers will need a valid license for Windows 7 or 8.x to upgrade to the final version of Windows 10.
@tao_huan Yes
— Gabriel Aul (@GabeAul) May 10, 2015