Reddit, often a first source of inside scoop, was at it again this morning, this time with Redditor betrion referencing a new Microsoft list that appears to indicate which Windows Phone devices will get updates to Windows 10 Mobile, and in what order. The source material is fairly dense, comprised of a WP 8.1 to 10 Device Upgrade Eligibility list in XML format, but parsing the data may reveal some interesting tidbits.
Apparently, Windows 10 Mobile will release in three batches, and as expected some devices won't get Windows 10 Mobile at all. It's also possible that some of the devices on the "never" list mean that Microsoft won't be providing the update and that it's up to the manufacturer.
Looking at the Microsoft list page, the XML document seems to be intended to feed data to an upcoming Windows Phone app. Waiting for that app to release before concluding anything might be the best option.
Supported Operating System
Windows Phone 8.1
none. used by WP app.
From what we can tell by going through all of the various information from a variety of sources (including NokiaPowerUser), here's how things look. And keep in mind that this is speculation at this point; the Microsoft document might not mean exactly what people are assuming it means.
Lumia 950 XL
Lumia 640 variants
HTC One M8 for Windows
Lumia 630 variants
Lumia 730 Dual SIM
Lumia 435 Variants
Lumia 532 Variants
Lumia 535 Variants
Devices that won't be getting Windows 10 Mobile, or that Microsoft isn't committing to update, include:
Samsung ATIV S
Dell Venue Pro
Samsung Omnia 7
Generally, it appears that Windows Phone 8.1 build 8.10.14219.341 is the basic requirement, along with 1.5GB of free space for the update. And, the list doesn't include every Windows Phone device out there.
We'll keep you updated on any more information that comes through, as well as keeping this list up to date if any clarification becomes available. Remember that this is all speculation, and that things are subject to change. Our advice: take this information with a huge grain of salt, and wait for more information from Microsoft before concluding that your Windows Phone device will or will not receive the update.