Difficult to say. My own desktop PC is an upgrade on top of 8.1 and I can't recall any particular issues with that. I've just installed 7 followed by 10 via the 'assistive technology' route onto my old laptop. I suppose if you have a much-used system which may have all kind of incompatible registry settings and programs set up for the earlier version you may have issues. But then again, most (all?) of the system files are stuffed into a Windows.old folder and replaced. That said, most of my installs have been clean.