Make a list of the systems current packages for later reference:

rpm -qa --qf '%{NAME}\n' | sort | uniq > ~/new-pkgnames.txt

Make a backup of any system configuration data (as root):

tar czf ~/etc-`date +%F`.tar.gz /etc


yum clean all

Make sure the new repo files isn’t placed as .rpmnew files, perhaps by

mv /etc/yum.repos.d/fedora-updates.repo.rpmnew /etc/yum.repos.d/fedora-updates.repo
mv /etc/yum.repos.d/fedora.repo.rpmnew /etc/yum.repos.d/fedora.repo

Also make sure that all the 3rd party repos you normally use point to the repository for the new Fedora release.

Next the upgrade.
Make sure you are in runlevel 3

telinit 3

Run the following command to update the yum repo on your box:

rpm -Uhv rpm -Uvh rpm -Uvh http://mirror.anl.gov/pub/fedora/linux/releases/8/Everything/i386/os/Packages/fedora-release-8-3.noarch.rpm http://mirror.anl.gov/pub/fedora/linux/releases/8/Everything/i386/os/Packages/fedora-release-notes-8.0.0-3.noarch.rpm

yum might complain about conflicts or requirements. That is probably because you have used non-standard repositories or installed non-standard packages manually. Try to guess which packages cause the problem (or at least is a part of the dependency chain) – uninstall them and try again. Remember to install the packages again if they are essential.

Tip: Find and review “lost” packages.
You can find orphaned packages (ie packages not in the repositories anymore) after the upgrade with the tool package-cleanup from the yum-utils package

yum install yum-utils; package-cleanup --orphans

It’s often helpful to run this command before the update, too. For packages with a failing “%postun” script the old package will remain partly installed. Remove it with

rpm -e package-name-and-version

If you forget to remove the avahi package it will refuse to be removed from the system with a error message “error: %postun(avahi-0.6.17-1.fc7.i386) scriptlet failed, exit status 1″. I haven’t looked up this error message any further.

I had to clean yum to make it aware of the recent package change, this might not be necessary.

yum clean all
yum upgrade

Ensure that all (new) essential packages from the new version is installed with

yum groupupdate Base

You might want to update other groups too, see

yum grouplist

The system had to upgrade 937 packages before fedora Core 7 is a fedora Core 8 installation.

After the upgrade has finished a reboot is required and it was time to check for orphaned packages

package-cleanup --orphans

The command for removing these packages is described earlier.

Source: http://docs.fedoraproject.org/install-guide/f8/en_US/sn-upgrade-tree.html, http://www.ioncannon.net/system-administration/133/upgrading-from-fedora-7-to-fedora-8-with-yum/ and http://fedoraproject.org/wiki/YumUpgradeFaq