I would be interested in this response as well.....my guess is that you would have to import your old projects into R9.0 to convert them, and then if all is good, you could get rid of the Logic 8 (or 7, 6, etc.....) if you never want to work with them again.
If you have partnered with people still working in R8.0 and older, you might want to hold onto those releases.
Again, this is all a guess on my part, but, I would still like to know if I got rid of those old versions to free up space (I use Logic on a MacBook Pro) that Logic 9 isn't affected at all.
Dennis
PS - Did a quick search on the forum, and couldn't find anything immediately.....but, I'm sure that if I took some time to dig some more, I might find something. So, apologies in advance if I am posting pre-emptively!