I ran the online upgrade process using preupgrade and apparently everything went smoothly upgrading to FC10.
The machine seems to behave itself and everything works as it used to, only when I run an update yum downloads packages marked with fc8 rather than fc10. So it would seem everything is not quite as good as it could be.
I was contemplating doing a format and install of FC10 to take advantage of the ext4 filesystem anyway. Next step - backup all those config files that I spent time working out (eg. httpd/svn/yum.repos.d etc).
Not sure when I'll get time to do it though........