Truth be told, I’m taking this with a grain of salt, especially after the T-Mobile Lumia 810 was left out in the cold back on Windows 8, but according to the official Lumia Twitter Page, “All Lumia Windows Phone 8 devices” will be upgraded to Windows 10 when it is released. Again, this is the same song and dance that Microsoft spouted about the move from Windows 7.5 to 7.8, then again on the move from 8.0 to 8.1, so I’m not going to put much into a passing tweet.
That said, we are almost a year away from seeing that magical update, so anything is possible. Of course, since this is the Lumia page and not the official “Windows Phone” or Microsoft accounts, there is no mention of other hardware manufacturers. HTC will give their standard, “We won’t comment on any rumors or speculation about future updates or devices” answer and Samsung doesn’t even respond to requests about Windows Phone device any longer.
As soon as there is official confirmation one way or another regarding any upgrade plans to Windows 10 for existing devices I will let you know what I hear, but until then, take posts and rumors like a grain of salt and realize that your contract will probably be up before Win 10 comes out anyways and you will probably want to get a new device either way.