During an update review, your existing published app and users will not be affected.

Your changes will take affect in the Development Environment of your app. Once you submit the update request and it is approved, then your Production Environment will be updated with the changes. Users will receive an email notifying them that there is an update, which they can click on to install / authorize the update if there is a scope change. If the users do not to update your app, it will still function, they just will not have access to the new features / scopes of your app.