"clean install" is the mantra of those who don't really know what's running on their Linux box.
The upgrade from FC5 to FC6 only needs downloading and installing the appropriate repository files (fedora-relese-6-4 and fedor-realease-notes-6-3), and then running
# yum clean all
# yum -y upgrade
Do the same to upgrade to Fedora 7 and Fedora 8.
Much easier to upgrade, then to re-install all needed applications, servers, etc and restoring all needed user and production data.