I finally upgraded my PowerStation from Fedora 18 to Fedora 21. The
upgrade went pretty smooth and was not much more than:

$ yum --releasever=19 --exclude=yaboot --exclude=kernel distro-sync $ yum --releasever=20 --exclude=yaboot --exclude=kernel distro-sync $ yum --releasever=21 --exclude=yaboot --exclude=kernel distro-sync

As I was doing the upgrade without console access I did not want to
change the bootloader from yaboot to grub2 and I also excluded the
kernel. Once I have console access I will also upgrade those packages.

The only difficulty was upgrading from Fedora 20 to Fedora 21 because
32bit packages were dropped from ppc and I was not sure if the system
would still boot after removing all 32bit packages (yum remove *ppc).
But it just worked and now I have an up to date 64bit ppc Fedora 21
system.