Microsoft just announced the new Windows 10 Mobile Redstone build 14291. Η συγκεκριμένη version preview is available for older Lumia devices, but is only distributed by program Windows Insider.
The previous available version of Window 10 Mobile was for devices that were pre-installed with Windows 10 Mobile, namely: Lumia 550, 650, 950, and 950 XL.
Build 14291 is the latest version of Redstone update and can be installed only through Windows Insider.
Microsoft has stated that the Redstone build will roll out to older Lumias after they upgrade to Windows 10 Mobile, and given the fact that the installation started last week, the company can now go ahead with the upcoming OS update.
The only device that does not seem to receive the new Windows 10 MOBILE Redstone build 14291 is Lumia 635, but Microsoft says it will be supported at some point in the future.
Unfortunately, unsupported phones are not allowed to install Redstone build, but they can continue to receive 10586.x updates, according to Gabe Aul, Microsoft. Devices such as Lumia 520 and 1020 are included in this list.
At the moment, no one knows about whether new devices will be supported at some point in the future.