It does not affect currently installed users -- you can only convert a brand new app with zero releases into an Unlisted app. You will need to migrate users from the old app (Custom app or Enterprise app) to the new Unlisted app. Unlisted Apps, once installed, update automatically just like normal public apps && custom apps. Unlisted Apps have a unique app store link that, when opened, reveals the app in the App Store app. It looks like any other app at that point.