The HTC HD2 unfortunately didn't receive the update to Windows Phone 7 although it had adept hardware configuration at that time. Thanks to the efforts of the developer community, the smartphone is still functional and supported with latest versions of different operating systems like Android Marshmallow, which was recently ported to the device.
It has been under a year since Lollipop was made available to the mighty HD2 with most features working and now the hackers from XDA-Developers have accomplished the same with Android's latest version. Strange thing is that, Marshmallow is currently not supported by quite a few new smartphones.
As per XDA-Developers member macs18max, the Marshmallow port for HD2, based on CyanogenMod 13 doesn't have compatibility with all the features and only the display, Wi-Fi and audio are functional. There's no information on its ability to access a cellular network anytime soon, which makes the build unusable.
The other operating systems which were ported to the HD2 in the past are Windows Phone 7, Windows Phone 8, Windows RT and Firefox OS, along with Android versions 2.2 Froyo, 2.3 Gingerbread, 4.0.4 Ice Cream Sandwich, 4.1 to 4.3 Jelly Bean and 4.4 KitKat.