I couldn't find the link but, as I remember, when you opt out you stay at the version you have installed until your beta version becomes the official release version. In other words, you can not downgrade and have to wait until the official release.
Edit: I found the site where I saw this info.
"If you leave the beta software program, the beta software will remain on your device until the final version of the software becomes available. Once the software is updated, you will no longer receive beta software."
