Microsoft has been backpedaling somewhat on its promise to deliver Windows 10 on “all” Lumia handsets, a statement that could easily have been (mis)interpreted at the time as “all Windows Phone 8.1 handsets.” But after affirming that it plans to provide the update even on low-end Lumias, a Microsoft executive this past week started resetting expectations yet again.

The issue here is obvious enough. While Windows 10 will run hardware as diverse as embedded Internet of Things (IoT) devices, smart phones and phablets, tablets of all sizes, laptops and detachable PCs, desktop PCs, and even Xbox One, it won’t offer the exact same features everywhere. And when you look at the Windows Phone market, or the Lumia market more specifically, only the lowest-end and least capable devices have sold well. Indeed, the Lumia 520 was the best-selling Windows device for calendar year 2013. And that handset was as low-end as they come when it was released, let alone today.

But promises are promises. Microsoft hasn’t yet divulged any details, but the company did admit to me during a campus visit last month that it would indeed be able to deliver Windows 10 to Lumias and other Windows Phone handsets despite its ongoing issues getting lesser updates out to phones. (Reference: Verizon Wireless v. Lumia Icon owners.) It will explain those plans at or before Build in late April, I was told.

Which leaves us with the obvious question: If Microsoft cannot or will not deliver a Windows 10 update to Windows RT users, what the heck will this update look like on low-end Lumias like the Lumia 520, 530, 625 or 630, each of which has just 512 MB of RAM?

It will look like … less.

In a series of tweets over the weekend, Microsoft’s Joe Belfiore tried to reset our expectations about what this update would look like, but also reaffirm that low-end users—i.e. 90 percent of all Windows Phone users—won’t be left out in the cold.

“We are working on Windows 10 for 512 MB devices, including the Lumia 520,” he tweeted (sort of: I’ve expanded this character-limited blurts into actual English for your convenience). “Our ambition is to make Windows 10 available for these devices but features may vary.”

“Timing for various phones may be variable as well,” he added. “When we first launch the Windows Insider program [for phones and small tablets], it’ll be [available for] a subset of phones with more [phones being added] later.”

“Also, we’re on track for Insider build coming “in February” as Terry [Myerson] said at our January 21 media event,” he wrote. “We need to complete internal testing first, [so] hang in!”

So this may seem like bad news, but compared to what’s happening with Windows RT—those poor guys aren’t even getting Windows 10—this is a much more reasonable situation, especially when you consider how much is missing from these low-end handsets. But then it makes sense for Microsoft to focus on the devices that sold (relatively) well. Windows RT just didn’t find an audience, and while Windows Phone is struggling compared to Android or iOS, Microsoft and its partners sell tens of millions of these devices each year.

Tagged with Windows 10 for phones and small tablets