Will your Windows Phone handset be able to make the leap to Windows 10?

Microsoft has been revealing which phones will be capable of running the mobile edition of Windows 10, and new information can be found on the company's website. A link called "Windows 10 Eligible" on the US Lumia phone website brings up a page that displays just five phones so far, namely the Lumia 640 XL from AT&T, the Lumia 640 also from AT&T, the Lumia 735 from Verizon, the unlocked version of the Lumia 635 and the Lumia 635 from Sprint.
But wait, there must more than five phones eligible for Windows 10. And yes, there are. But the question of which ones may not make the cut still seems to be unanswered.
Due to roll out later this year, the mobile edition of Windows 10 will offer several changes and enhancements for users, including a new and more appealing design, a new and improved Action Center that allows you to fine-tune notifications for different features and tasks, a new settings menu and the Microsoft Edge browser. With its mobile market share less than 3 percent, Microsoft is struggling in the mobile arena and is looking for help from Windows 10. The new OS will offer a similar look and similar apps across all devices, from PCs to tablets to phones, a strategy Microsoft hopes will attract more interest from both PC and mobile phone consumers.
So, just which phones will support Windows 10 beyond the five listed on Microsoft's website?
The UK version of Microsoft's site lists a few additional phones, including the Lumia 830 and the Lumia 435. In March, Microsoft released a lengthy list of current Windows Phone handsets that it said were capable of supporting the second test build of Windows 10 Mobile. And in November 2014, a tweet from Microsoft's Lumia Twitter account said that "there will be Windows 10 upgrades for all Lumia Windows Phone 8 devices."
So why are there only a few devices listed on Microsoft's "Windows 10 Eligible" page? That section of the site could be a work in progress. Or it could be in error. Either way, with Windows 10 Mobile coming out this year, Microsoft needs to confirm for sure which phones will be supported and which ones won't. Users of Windows Phone want to know if they can stick with their existing phone or will have to upgrade to a new one to receive the benefits of Windows 10.