There were upgrade specials when 10 was first released. I believe emails were sent out to registered users. If there are any now, I don't know. I suggest you contact Mark and ask. The worse thing that will happen is he will say no.
10 is a totally separate package and it is not only possible but often done to have 9 and 10 both running side-by-side. I rarely use 9 except to try to answer specific questions but I do still have 9 myself.