News suggesting that Microsoft plans to offer Windows 10 upgrades for all its Windows Phone 8 devices broke today. That would mean any Lumia smartphone that runs Windows Phone 8.x would eventually get the latest and greatest operating system from the company. Sadly, that’s not completely true.

A November 11 tweet from the Lumia account was picked up Windows Central, followed by other publications as Windows Phone owners got excited:

We contacted Microsoft to confirm or deny the answer its support team gave. It turns out the company’s stance right now (remember, it hasn’t detailed much about Windows 10 for smartphones just yet) isn’t crystal clear.

“It’s our intention to enable a Windows 10 upgrade for Lumia Windows Phone 8 smartphones,” a Microsoft spokesperson told VentureBeat. “At this early stage in the development process, and given the vast portfolio of Windows devices worldwide, we can’t predict that all devices will be upgradeable, but it is our intention that the Lumia smartphone line be upgradeable to Windows 10.”

It’s important to note the Windows Phone 8 distinction, because Nokia’s first Lumias were running Windows Phone 7. Only Windows Phone 8 and Windows Phone 8.1 devices stand a chance for a Windows 10 upgrade. Yet that’s still much better than what happened with Windows Phone 7: None of the first devices could be upgraded to Windows Phone 8 because of a kernel change Microsoft had to make.

The fact that Microsoft can’t promise the upgrade for all devices is no surprise. If a manufacturer doesn’t want to support its smartphone past Windows Phone 8, that’s the manufacturer’s decision to make (we doubt all of them will shun Windows 10). As for Microsoft’s own devices, the situation is probably more nuanced.

Some Windows Phone 8 Lumias are over two years old now and will be even older when Windows 10 arrives later next year, so there could be technical limitations. Even if there are no special hardware considerations involved, carriers have their own priorities: to sell more devices.