Microsoft is currently working on the Windows 10 for phones update for devices running 512MB RAM.
Microsoft's Joe Belfiore has reasserted this through a tweet, which Gabriel Aul confirmed last week. While the list of 512MB devices is yet to be out, Joe Belfiore said Lumia 520, the popular mid-range Lumia handset launched in 2013, is one among them.
Unfortunately, all features from Windows 10 for phones will not be available for these devices.
He also stated that the first technical preview build accessible through Windows Insider program, to be launched later this month, will not be available for all Windows Phone devices. Instead it will only be for a subset of devices, although support for more devices will be followed later on.
This rollout will be unlike earlier Windows Phone developer previews, which were released for all devices simultaneously.
The Windows 10 for phones preview build is currently undergoing internal testing, added Belfiore. However, Microsoft is still on track to bring out the Insider build this month, which was announced during the January 21 event for Windows 10.
The preview build for the Windows phone version might come as 'Windows 10 Mobile Technical Preview', as spotted in a recent leak of OS version 8.15.12482.4.