Switching from User to Device App Assignment on iPads

As long as the app stays scoped in the JSS app catalog, the JSS shouldn't send out uninstall commands(to the best of my understanding). However, the app may uninstall itself when the license is revoked. The choice of the app removing itself or not is made by the developer of the app, so every app is different.

iCloud backups with device assigned apps are interesting. When restoring the backup, depending on how many licenses there are to assign and how quickly those licenses get assigned, some-all of the apps will fail to install because the license isn't present. However, even if the app doesn't reinstall automatically after the restore, the data connected to those apps will restore(usually) and you should see that data in the app after installing it from Self Service.

Side note: You should check the "Prevent backup of app data" box for Self Service to prevent the iCloud backup from restoring app config data that contains the wrong device information.