Despite an earlier commitment to upgrade all Lumia Windows Phone 8 devices to Windows 10, Microsoft now says some phones will be left behind.

"Like any upgrade to a new platform, not every phone will upgrade or support all possible Windows 10 features, and certain features and experiences will require more advanced future hardware," Microsoft wrote on its Lumia Conversations blog. "Our goal is for the majority of the Lumia phones running Windows Phone 8 and 8.1 to join the Windows ecosystem along with an expected hundreds of millions of PCs, tablets and other devices running the next generation of Windows."

That statement runs contrary to a post from Microsoft's Lumia Twitter account in November, which said upgrades would be available for "all Lumia Windows Phone 8 devices." So far, there's no word on which devices won't make the cut, and which features may not work on older hardware.

Windows 10 for smartphones isn't a major departure from Windows Phone 8, but adds many useful features such as a floating keyboard for one-handed use, universal speech-to-text, Skype integration in the messaging app, actionable notifications, an expanded list of quick settings toggles and a full-featured version of Microsoft Office. It's expected to arrive later this year, with a Technical Preview launching in the coming weeks.

Why this matters: Microsoft's track record for upgrading Windows Phones is already spotty, having abandoned all Windows Phone 7 users with Windows Phone 8, and allowing a handful of carriers to stall the Windows Phone 8.1 upgrade for certain phones. But at least in those cases, Microsoft had excuses: Windows Phone 8 switched to a new kernel that was incompatible with the previous version, and carrier holdups aren't really Microsoft's fault. In the case of Windows 10, Microsoft simply made a promise it couldn't keep.