At its press event yesterday, Microsoft showed off Windows 10 running on its Lumia smartphones for the first time, and the company has promised to release its first preview of the OS for phones sometime next month.

Microsoft confirmed this week that all Windows Phone 8.x devices will be eligible for free upgrade to Windows 10 when it launches later this year. But today, it elaborated on what that commitment means with a couple of important points.

Chris Weber, Corporate Vice President of Sales for Microsoft's Mobile Devices group, said today that the new OS "has been designed to run well on today's Lumia phones." But he added that "not every phone will upgrade or support all possible Windows 10 features, and certain features and experiences will require more advanced future hardware."

In practice, this seems like common sense. The first Windows Phone 8 devices are now over two years old, and simply aren't equipped to support every single feature that the far newer Windows 10 OS will offer. Really, this comes as no great surprise, given that Microsoft recently confirmed that its older and cheaper handsets will not get new firmware with the Lumia Denim update that's now rolling out.

However, Weber also said (emphasis ours): "Our goal is for the majority of the Lumia phones running Windows Phone 8 and 8.1 to join the Windows [10] ecosystem", which implies that some devices may get left behind.

This may be a reference to handsets that we already know about, like the Lumia 810. Exclusive to T-Mobile in the US, the carrier dropped support for the device after just 16 months on sale, and refused to offer any further OS updates for it. But the possibility remains that other devices may join the 810 in also being unable to update to Windows 10.

Still, if that does turn out to be the case, it won't be the end of the world for owners of those handsets. Microsoft's Windows Insiders program offers access to the latest OS versions before they're officially released, and with the first new Windows 10 build on the way for phones in a few weeks, it seems that Microsoft will continue to offer this lifeboat for those whose carriers have dropped support for their devices.

Unsurprisingly, Microsoft's message to its customers remains that "there's never been a better time than now to buy a Lumia!" - but if you want to take advantage of all the very best features in Windows 10 on your handset, you might want to delay your purchase until the company unveils some new hardware. Microsoft confirmed yesterday that it is working on new 'flagship Windows phones' that will launch later this year.