A few days ago I started to upgrade my PowerStation from Fedora 15 (running my own rebuild) to Fedora 18 Beta.
The update from the running Fedora 15 to Fedora 16 was the really hard part. It seems that the userspace moved from 32bit to 64bit and that was something that yum, understandably, could not handle. So after the first run of all packages updated to Fedora 16 (which required a lot of rpm -e --justdb --nodeps --noscripts) and a reboot the system was broken. systemd tried to start udev but that failed with:
[ 38.164191] systemd: udev.service holdoff time over, scheduling restart.
[ 38.208255] systemd: Job pending for unit, delaying automatic restart.
and systemd kept printing those lines forever. Luckily I still had the original Yellow Dog Linux installation on a second drive and could boot that. Unfortunately I could not chroot into the Fedora 16 installation because the Yellow Dog Linux kernel was too old, but I was able to mount it and disabled every occurrence of udev in systemd. Rebooting with
systemd.unit=emergency.target on the kernel command-line I was able to get the network running and reinstalled with yum the udev and systemd ppc64 packages. After that (and some more fiddling around) it rebooted into Fedora 16.
I then just followed the recommendations on the Fedora wiki to upgrade using yum from F16->F17 and F17->F18. The only difference was that I installed the gpg key, which is used to sign the packages, from https://fedoraproject.org/keys using the keys for the secondary architectures.
Now I have a PowerStation with the latest 64bit Fedora 18 Beta packages up and running.