The first Windows 10 preview build for phones was released over a month ago, but it disappointed some enthusiasts because it was only compatible with a few recent midrange Lumia phones. Owners of the Lumia 630, 635, 636, 638, 730, or 830 could install the update, but older hardware and flagships were excluded. The good news is that the next update will greatly expand the list of supported devices—according to a blog post from late yesterday, the vast majority of past and present Lumia phones should be able to install the new preview build when it lands.
The first preview build for phones didn't install on most of these Lumias because of the way their internal storage was partitioned. Newer phones had large enough partitions to install the update, but older phones apparently didn't. A "partition stitching" tool that can resize partitions without destroying the data on them was recently finalized, and it will let users of these phones install the update without completely wiping their phones.
Notably absent from the new list are any phones from third-party OEMs, regardless of age or specifications. This is still preview software and these are still early days, but owners of the handful of Windows Phones available from HTC, Samsung, Blu, or other OEMs will have to be content with Windows Phone 8.1 for now. Any phone that can run Windows Phone 8 can technically run Windows 10—these devices range from low-end Snapdragon 200 handsets with 512MB of RAM all the way up to top-tier phones with high-end SoCs and 2GB of RAM—but at the moment the phones that are the most likely to receive an update are all Lumias.
We don't know when we'll get the next Windows 10 build for phones, but when it arrives this is the full array of handsets that should support it. This list is still subject to change, but absent any showstopping bugs for specific devices most post-Windows Phone 8 Lumia owners should be covered.