The new preupgrade tool, available as a package on both Fedora 8 and 9, allows you to download and install all the necessary rpms required for upgrading from Fedora 8 to 9. This tool not only can save on media downloads, but also allows you to continue to use Fedora 8 while all the packages are being downloaded and prepared for Fedora 9.

Overall, it works great. Once the packages are downloaded you reboot into anaconda, the Fedora installer, and a normal system upgrade/install takes place with all the downloaded packages. However, there are a couple of caveats. Firstly, it only checks rpm dependencies and downloads from the fedora repository. Third party repositories (such as livna) are not included. Secondly, it is a good idea to clear your yum cache immediately after the reboot and install:

This will prevent some confusing package management with both the main and third party repositories once Fedora 9 is installed.