I will attempt to shed light on what I can here, as it seems many of you are concerned and have problems, not without good reason in most cases.
The first question I will answer is the matter of "will I lose my old programs/data/etc". The answer to that is if you upgrade, without doing a clean install, you shouldn't. The installer will prompt you before the main process begins (after it makes sure everything is ready/compatible), saying "a few things need your attention" in a section of that dialogue. There you will find, if no apps are incompatible, an install, refresh, back and maybe some other buttons that I can't remember off the top of my head. If you have an app such as older versions of Jaws, they are considered incompatible by the installer (newer versions will not be affected if you remove these), the install button won't display. instead, you'll be able to uninstall right from within the dialogue box itself and carry on, whether that's with the uninstallation of further apps or the rest of the installation of the new OS.
The second point I'd like to address is the idea that Microsoft don't give any thought to accessibility. I would say otherwise, simply due to the fact that you can give feedback to Microsoft and interact with the rest of the community, via the windows 10 feedback app. I haven't tried it myself in the public launch version of the OS, but when I got a chance to test it (which, granted, was a while ago during the preview builds), it seemed accessible, if a little slow.
The third point I feel needs addressing is that of using narrator in the installation. After the aforementioned compatibility dialogue, once you press install and wait a while for the OS to physically be installed onto the drive, you can press windows u to bring up the ease of access centre that will speak and then start narrator from there. You can then read all the controls in the setup dialogue. The one thing I will alert you of now is that when you finish this set up process, the installer will keep telling you that it is "preparing windows" and you are told "do not turn off your PC". Once that's done, NVDA should load and you should be good to go.
One final thing for now, the compatibility assistant will say that NVDA is not compatible when in fact it'll work under windows 10 with pretty much the same accessibility as before, other than a one addon that I've found not to work which is the virtualisation addon. This might be fixable, I can't say for sure but someone somewhere will probably find a way round it.
If I think of anything else that needs to be noted, I'll post it here.
Regards,
Sightless Kombat.
***If you wish to refer to me in @replies, use Sightless***