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 users in making most out of their Windows Phone Phablets mayn’t be available for smaller screen devices. Though, we are not very sure of the minimum display size criteria, but it may be 4.9 inch or higher.
Coming to when one can expect Windows (Phone) 10 to hit developer preview devices, sources talk about a April-May 2015 timeline now.