Microsoft working on a way to update low-end Windows Phone handsets with parts of Windows 10
This means that the most popular Windows Phone model of all time, the Nokia Lumia 520 with over 12 million units active as of last July, would be updated. However, before you get too giddy, Belfiore's tweet hinted that these low end models would receive only certain features of the Windows 10 update. Belfiore also sent another tweet adding that the timing of the update will vary among devices. He said that when the insider program is launched, it initially will support a "subset of devices" with the program broadening out as time goes on.
And the Microsoft executive sent out one last tweet stating that the Insider build of Windows 10 for Phones is still expected to be available this month. Right now, the OS is undergoing "internal testing."
source: @joebelfiore via WMPoweruser