Microsoft plans to let Windows 10 preview testers upgrade to the RTM versions at no cost when it releases the OS sometime in July even though Microsoft already said they’ll allow free upgrade for Windows 7 and 8 users. In a PC World post, Microsoft’s Gabriel Aul wrote that that Insiders will be able to upgrade to the RTM (Release to manufacturing) version at no charge. This is the version that PC makers load onto their machines before shipping them out to consumers.
Usually, testers of a software of this magnitude might be concerned about the final product and how it may mean that they may be required to provide a license later. In this case though, that’s been covered by Microsoft with an upgrade to the final OS for free.
You can still preview the Windows 10 here