What if the app is updated and the backup data is not working?
If an app is updated enough to require changes to the data, the developer already has to provide a way to migrate it on first run, because otherwise every user who updates the app loses all their data.
How the app can know that the data is restores? when you install an app, the app re-create a new isolated storage.
Restore only happens once - right after setup of a new device. The app wouldn't need to know if data is restored - either there's data there and it uses it, or there's not and it creates its own.
And the bad news is that the 150,000 apps (or whatever that number is) that currently exist will not for now (and possibly never, depending on how many of the developers are still actively doing anything with their apps). The ~10% of those existing apps that are games? Likely to never support data backup. Users don't care which type of app it is (whether WP 8.1 Silverlight, WP 8.0 Silverlight, XNA, DirectX, whatever)... they just all of want their data to still be there when they lose or upgrade a phone. Windows Phone is the ONLY platform that doesn't have any solution for this, even still.The good news are that WP8.1 apps will have the options for cloud backup.
