Martin, thank you for replying!
Actually I meant not the active installations of Cubase, but the installable packages, which I downloaded locally from Steinberg site - should I keep all of them for future reference, or only 10.5 is sufficient? If in the future I want to re-install Cubase, I guess I could use only the 10.5 installer, without first having to install 9.5 and 10.0?
Otherwise, the other question is also important, but I solved it so far by uninstalling 9.5 Artist/AI and going to a fresh full 10.0 (actually dir even fresh OS re-install), and now installed only the 10.5 update. As I got two Cubase active installations now though, at some point I may uninstall everything and go with the 10.5 full installer.