MSDN

According to the ChangeLog on July 29th all Windows Insiders signed in with an MSA account keep their copy of Windows 10 activated free of charge.

"Windows Insiders running the Windows 10 Insider Preview (Home and Pro editions) with their registered MSA connected to their PC will receive the final release build of Windows 10 starting on July 29th. This will come as just another flight. I’ve gotten a lot of questions from Windows Insiders about how this will work if they clean installed from ISO. 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."

http://blogs.windows.com/bloggingwindows/2015/06/19/upcoming-changes-to-windows-10-insider-preview-builds/