In the past week, Windows 10 Mobile devices, which are the Lumia 550, 650, 950 and 950 XL as well as the Xiaomi Mi4 have been able to upgrade to Redstone preview builds 14267 and 14267.1004 providing users of those phones are enrolled into the Insider Program and have opted for the Fast Ring. Microsoft even added support for the Alcatel OneTouch Fierce XL yesterday, allowing it to also receive these latest builds.
Currently, all Windows Phone 8.1 devices that are supported in the Insider Program can only get Windows 10 Mobile build 10586.107 (which is a Cumulative update) on the Fast, Slow and Release Preview Rings, leaving those users that have opted to be fast tracked for previews out in the cold. Earlier today Gabe Aul made it a bit clearer on why this is happening.
It now appears that anything other than the newest Windows 10 Mobile devices will have to wait to start testing Redstone builds until the retail version of Windows 10 Mobile has been rolled out to those devices; and depending on your carrier, this could take a while, as the rollout has already been delayed a number of times.
Not such good news for those eager to test the latest and greatest; but If you are thinking of doing a registry hack (which some people have done already) to get Redstone builds, Gabe advises against this, warning people that it could turn your phone into a very expensive paperweight.
So it looks like we will have to wait until Microsoft and carriers get their fingers out and start pushing Windows 10 Mobile to 8.1 devices, that is, if we are to take what Gabe has said literally.
However, there is a bit of hope that the wait won't be too long, we learned a week ago that Microsoft Mexico spilled the beans on the date that Windows 10 Mobile would start rolling out for Windows Phone 8.1 devices, and that is February 29, this coming Monday.