Today, Microsoft roll-out Windows 10 Mobile to select Windows Phone 8.1 devices. There are a lot of great new features in Windows 10 Mobile, like Continuum, Windows Hello and Cortana.
The current list of Windows Phone 8.1 devices that are supported for the upgrade to Windows 10 :
Lumia 635 1GB,
Lumia 636 1GB,
Lumia 638 1GB,
BLU Win HD w510u,
BLU Win HD LTE x150q,
MCJ Madosma Q501
This list includes a limited set of Windows Phone 8.1 devices and does not include some devices that are currently running Windows 10 Mobile in the Windows 10 Mobile Insider program. As Windows 10 delivers significant new innovations, many older devices are not able to successfully upgrade without an impact on the customer experience. Our goal is to only offer the Windows 10 upgrade to devices that we are confident can continue to deliver a good customer experience.
For more information on Windows 10 for phones, go Here