Four months after the first phones with Windows 10 Mobile preinstalled shipped, Windows Phone 8.1 devices can now officially be upgraded to Windows 10 without the use of Insider/Preview builds.
Unfortunately, not all Windows Phone 8.1 devices will be upgradable. Some phones, mostly (though not exclusively) older models, aren't going to be upgraded. Frustratingly, this includes some phones that were previously eligible for the Windows Insider Program, meaning that they could run the previews and betas but aren't going to get the final stable release. Handsets including the Lumia 920 and HTC One M8 both fall into this awkward category. It's possible that these phones will be added later.
For now, the list of supported phones is: Lumia 1520, 930, 640, 640XL, 730, 735, 830, 532, 535, 540, 635 1GB, 636 1GB, 638 1GB, 430, 435, BLU Win HD w510u, BLU Win HD LTE x150q, and MCJ Madosma Q501.
The upgrade from 8.1 to 10 is also subject to the usual carrier approval and annoyances, so even some models that may technically be upgradable may not be upgradable right now. To find out if your phone can be upgraded, Microsoft has an upgrade advisor app that will reveal if your handset makes the grade or not.
Windows 10 Mobile is a major update to the platform that's an instrumental part of Microsoft's Universal Windows Platform strategy. But with the market share at an all time low, it's not clear how effective this will actually be.