The crux is you have two choices (plus one):

• Stay with the inside Program and get Win 10 for free – Continue as a Windows Insider and keep getting preview builds after 7/29 – This will keep you on a Preview OS, but even if you didn’t have a previous Windows install (version 7 or 8.1).
• Or upgrade your Genuine Windows 7 or Windows 8.1 system that has been getting Windows 10 Insider Preview builds to the 7/29 release and stop being an Insider – This will keep you on the released build of Win 10.
• Re-install your previously licensed Windows OS (version 7 or 8.1) – If you didn’t have one, your Windows days are over.

Details after the image below.

“I want to continue as a Windows Insider!” 

If you want to continue as a Windows Insider past 7/29 there is nothing you need to do. You’re already opted in and receiving builds in the Fast or Slow ring depending upon your selection. This is prerelease software and is activated with a prerelease key. Each individual build will expire after a time, but you’ll continue to receive new builds so by the time an older prerelease build expires you’ll have received a new one. Since we’re continuing the Windows Insider Program you’ll be able to continue receiving builds and those builds will continue to be activated under the terms of the Windows Insider Program. We provide ISOs for these builds for recovery from any significant problems, but they are still pre-release software. As part of the program we’ll upgrade Insiders to what is for all intents and purposes the same build as what other customers will get on 7/29, but that will be just another build for Insiders, and those who stay in the program will simply get the next build after as well.

“I want to opt out of the Windows Insider Program on 7/29.” If you decide to opt-out of the program and upgrade to the 7/29 build you will be subject to exactly the same terms and conditions that govern the offer* that was extended to all Genuine Windows 7 and 8.1 customers. This is not a path to attain a license for Windows XP or Windows Vista systems. If your system upgraded from a Genuine Windows 7 or Windows 8.1 license it will remain activated, but if not, you will be required to roll back to your previous OS version or acquire a new Windows 10 license. If you do not roll back or acquire a new license the build will eventually expire.

Editorial Note: My Win 10 experience has been pretty solid, (even during the preview) – I think I will stick with the preview.

Link Reference – Microsoft Windows Blog