It is not a hidden secret that Microsoft plans to unify Windows, Windows Phone and Windows RT in one Windows with the upcoming Windows Threshold or Windows TH. This will make it even drop phone from Windows Phone going forward. We have seen some evidences of "Windows" branding on phones.
For those asking, Windows (Phone) 10, the mobile / laptop SKU of one Windows Threshold should be available for all existing Windows Phone 8 devices including Lumia 520, as far as we know. Though, not all features will come to all the devices!! Some features like Split-screen multitasking, which will help