WP9 to be built from scratch and have new UI (from Mobile Review)
According to Eldar Murtazin of Mobile Review, the next version of Windows Phones will be both a tablet and smartphone operating system. What's more, Murtazin says we'll see a whole new UI that will be different to Metro UI(though, he notes, Microsoft won't actually be able to admit mistake with Metro, as it's been pushing the interface so actively in advertising). He says the new version will be developed almost from scratch.
I wouldn't be surprised at this move. iOS 7 is a big departure from the original iOS and Honeycomb started a whole new UI for Android. WP9 will probably follow in the same footsteps and break from the previous iterations of WP.