While we were the first ones to report that Microsoft will not offer Windows 10 to existing Windows RT devices like Lumia 2520, it seems there will be an update coming for these devices in September.
As per Gabe Aul, Windows 8.1 RT Update 3 will be available in September. The changelog is not known yet but the upcoming update may bring these devices closer to Windows 10 UI wise.
@jon5798 Windows 8.1 RT Update 3 will be available in September
— Gabriel Aul (@GabeAul) July 15, 2015