This isn't 100% correct. Your "go-back" option is directly related to what you can go back to. For example, if you have Windows 7, and then used the free upgrade and you went to Windows 10 build 10240. Within 30 days you can revert back to the previous OS. If within the 30 days, Windows 10 updated to 10586, the "go-back" then would not return you to Windows 7, but the previous build. IF the option even shows up anymore. And I believe that in order to return to Win7, the windows.old folder needs to remain. I do not know if the upgrade to 10586 replaces this folder, or even if you can "chain" go-backs. These build numbers are (real) examples, but it more correctly relates to any updates where Windows did the in-place Upgrade to install it. So originally, the 30 day thing was correct. Now it is, 30 days unless you install an update. I have read a few times that people have upgraded and had no button to go back while still within the 30 day window, so it is possible that something else can make it go away, such as a regular Windows update.