Microsoft took a fresh approach when designing Windows Phone 7. The operating system was a complete departure from Windows Mobile 6.5 and though Windows Phone 8 shares a lot of the same design elements as Windows 8 for desktop, it doesn't actually run the same apps as Windows Phone 7. Now it looks as though Microsoft is ready to redesign mobile once again.
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.
If you've been eying up one of the current Windows Phone 8 devices and are now panicking about it being out of date, you needn't worry. Murtazin notes that this OS doesn't yet exist and will likely only be completed in 2015.