When doing so, note that you first upgrade the Bubble engine version for your app's development version, which then must be deployed in order for this to apply to your live app. Thus, if you upgrade in your development version but change your mind, you have the opportunity to revert back in your development version - as long as you have not deployed to live yet, your live version will not have been impacted. However, you will not have the option to revert to a previous Bubble engine version forever, only for a very short time window after you upgrade.