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If you're planning on getting an iPhone 11, iPhone 11 Pro, or iPhone 11 Pro Max on launch day, you may run into an issue if you've been on the iOS 13 beta track this summer and plan to restore your new iPhone from a backup. If you updated to the iOS 13.1 beta when Apple released it in August, the backups you've made since then have all been on iOS 13.1. This is a problem if the phone on which you've installed the beta is also the phone from which you plan to restore your new iPhone, which, by all indications, ships with iOS 13.0.
Here's what you'll need to do to restore your new iPhone 11 from a backup of a phone on the iPhone 13.1 beta.
The problem
Because iOS 13.1 is a later version of iOS than 13.0, backups, whether to iCloud or iTunes, can only be restored to devices running iOS 13.1 or later, and are not backward-compatible with iOS 13.0.
Because iOS 13.1 won't release to the public until September 30, the iPhone 11 lineup will initially ship with iOS 13.0. This means that you won't be able to restore your iPhone 11 from a backup of, for instance, your iPhone XR if said iPhone XR has been running the iOS 13.1 beta.
While those on the developer beta have quite possibly been using the iOS 13.1 beta on a test device, it's quite possible that those users on the public beta track have installed iOS 13.1 on their primary (or only) iOS device. If you're someone that is running the beta, public or developer, of iOS 13, and are planning on upgrading to an iPhone 11, then this is for you.
The solution
The answer to this problem is, unfortunately, fairly convoluted, and depends on whether or not Apple releases beta versions of iOS 13.1 for the iPhone 11, iPhone 11 Pro, and iPhone 11 Pro Max.
As I see it, there are two possible scenarios.
Scenario 1: Apple doesn't release a beta
This is actually the simplest one. If Apple doesn't release a beta version of iOS 13.1 for its new phones, then that's that. You can either set up your iPhone 11 as a new iPhone, forego restoring from a backup, and update to iOS 13.1 on September 30, and maybe restore from a backup then.
Or you could hang on to your old, iOS 13.1-running phone and use that for another 10 days until the official release of iOS 13.1 to the public, at which point you can then set up your new iPhone 11.
Scenario 2: Apple releases a beta in the afternoon of iPhone launch day
This is more complicated, but it's how Apple has done things in the past. At some point on iPhone launch day, Apple should release a version of the iOS 13.1 beta for the iPhone 11 lineup. This should be available to both developer and public beta users, but you'll have to play it by ear.
Whatever the case may be, in order to upgrade your new iPhone to the beta, you'll need to activate it as a new iPhone first. Once you've set it up, enroll it in either the developer or public beta program (you'll need a developer account to do the former). Once you've done that, you can download the beta over-the-air as soon as it's available.
After the upgrade: Erasing your iPhone
Assuming Apple releases an iOS 13.1 beta for the iPhone 11, once you've upgrade your new phone to it, you'll want to erase it if you want to restore a backup from your previous iPhone. That's easily accomplished by a trip to Settings.
How to wipe all personal data and erase your iPhone and iPad
Restoring your iPhone from a backup
So, to recap: you've set up your iPhone 11, updated it to the iOS 13.1 beta, and now you've erased all of it. Once erased, your iPhone will still be on iOS 13.1. Now you can restore it from an iOS 13.1 backup of your older iPhone.
How to restore your iPhone or iPad from a backup
Once restored, your new iPhone 11 will be on the iOS 13.1 beta, and you can update to the release version of iOS 13.1 on September 30 when it launches to the general iPhone-owning public.