Pretty much all of the WP8 Lumia smartphones are compatible with the Preview version of the upcoming Windows 10 OS. Many people have tried the test builds and helped shape the final version of Windows 10, which should kick off later this year.
There are lots of Windows Phone devices outside the Lumia lineup, but until today just one got the Windows 10 Preview treatment - the HTC One M8 for Windows. Today's rumor suggests another phone will be added to the list.
As the report has it, the HTC 8X will be also getting access to the Technical Preview for Windows 10. It will be also included in the Windows Phone Recovery tool. The compatibility update should become available soon.
All Windows Phone 8 smartphones will receive an update to Windows 10, but merely a fraction will get to try the Preview builds.