Expect a Limited iOS 11 Downgrade
If you download the iOS 12 update on release day and you start running into major bugs or performance issues, you should have the ability to downgrade to iOS 11.
Apple should be signing off on iOS 11.4.1 on release day which will allow you to drop your phone or tablet back down. Unfortunately, this option won't be available forever.
At some point Apple will permanently close off the iOS 11 downgrade. Once this happens, you'll be stuck with iOS 12 going forward.
Apple won't announce when it plans to close off iOS 11, but you can expect the downgrade to stay open for a couple of weeks after iOS 12's release date.
If you're unfamiliar with the downgrade process, now would be a good time to look into it.
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