Some Windows Phone users were disgruntled about missing out on the first test build of Windows 10 Mobile. Microsoft is fixing that adding support for phones besides the Lumia 630, 635, 636, 638, 730 and 830.
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Apparently the reason so few devices were supported was because too many phones had insufficient room to update the OS. The second test build, coming in April most likely, will not have the issue. Microsoft execs attributed the small number of initially supported devices to the "very tight OS partitions" that left insufficient room for the installation process to update the OS in place.
Expect the test build to have bugs as it is after all a beta, with Microsoft hoping for helpful feedback before the official release. Looking at the list, it's pretty comprehensive with models that include the Lumia 920, 925 and 928 but leaving out the 930 due to bugs that still need to be ironed out. The 500-series Lumias are also on the list as well as the Lumia 1020. There might be changes as the launch date nears
For a full list of supported phones, check out the blog post here.