You will read the rollback stories because this is only a technical preview of what is to come. Lots of testing, trial and error for Microsoft using a bunch of us willing Guinea pigs for the "greater good." Since MS has blurred the lines between Alpha, Beta and RTM calling this a preview, who knows if or when their will ever be a time when we get to Windows 10 and stay there with the existing handsets.
My personal feeling is Microsoft has no intent on making us whole with Windows X for phones on existing hardware. If anything, we'll get to "good enough" aka Windows Phone IX but to get the full breath and depth, we have to buy new equipment. They did this before when Windows Phone 7 hit end of life. We got some skin on our phones that mimicked version 8 but it was far from version 8.