I've also posted this problem on Apple Discussions and was offered another suggestion. I archived my Safari bookmarks and Mail boxes (I have several - organizing, etc.). Then I also checked Applications, documents to see if there were any differences, and if so, copied them. (There really was very little difference.) Then I rebooted to 10.5.8, imported the bookmarks and mailboxes. Now that everything is up to date, I can boot from the 10.5.8 drive for a few days more, just in case I forgot something, and then do a reverse clone with Superduper.
My reason for wanting to "go backwards" is a problem I'm having with 10.6 - a "sudden blue screen reboot". Read more here if you're interested -
http://discussions.apple.com/thread....art=0&tstart=0
Thanks again.
RGreen