@yogesharya2011 Yes we believe this is solved now with a change to WPRT and will offer again for 520s soon (likely late in the day tomorrow)
— Gabriel Aul (@GabeAul) April 20, 2015
Lumia 640 should also be able to get Windows 10 for Phones update now.
@WernerKru @joebelfiore 640 should be updateable now. Make sure you are in the Fast ring in Windows Insider app.
— Gabriel Aul (@GabeAul) April 20, 2015