Thursday 30th July 2015: 12.54am. Just spent the past hour trying to turn my Windows 10 Insider Preview into a Genuine Windows 10 RTM without keylogger and Microsoft spyware sending everything you do to Microsoft. The Insider ISO was installed about a month ago as a full system wipe as my previous Win 8.1 was 32 bit, and I wanted 64 bit on this laptop. About a week ago I got the build 10240 which is supposedly RTM.
It turns out it is so incredibly easy I wasted an hour on trying to verify if it worked or not. You simply enter Settings => Update and Security => Advanced Options and click the button which says "Stop Insider builds". You'll need to agree to a EULA, it'll go fetch a new product key from Microsoft and will reboot your machine.
On reboot, I used the Magical Jelly Bean KeyFinder to pull the new product key. It looks to be unique, and legit, and definitely not the product key used by Insider Previews. Which suggests my Windows 10, despite not being an upgrade from a valid licensed Windows, is now fully genuine and activated and licensed for free.
I did learn during my searching of Google this past hour that I am not alone in this experience - others have had free copies of Windows 10 too. However, some did not get this experience, and have ended up with Windows 10 installs which are non-genuine and pester you for a paid product key. So I guess you takes your chances, and if it doesn't work out oh well you can reinstall your previous legit Windows and do the upgrade from there - equally if it does work, you just saved yourself a ton of hassle.
Is this "free" copy actually time bombed and will expire soon? I have no idea. I guess there is only one way to find out ... still, for anyone else in a similar position to me, give the above a try, maybe it works for you too.
Oh, and once you're off the Insider Programme, don't forget to go into the settings and disable all the keylogging and spyware stuff. Those options were greyed out in the Insider Preview, they should now be available to be disabled.