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.