You Can Only Downgrade to iOS 9.1
If you want to move to the iOS 9.2 beta, you'll want to be extremely careful, especially if you're coming from something older than iOS 9.1.
Yesterday, Apple plugged up the iOS 9.0.2 downgrade which means that if you do upgrade to iOS 9.2 beta, you'll only be able to drop down to iOS 9.1, nothing older than that unfortunately.
It will stay like this until Apple releases a new iOS update so you'll need to be extremely confident in iOS 9.2 (and iOS 9.1) before you take the plunge with the beta.
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