Sorry, couldn't resist. =)
So normally I do my updates from one version of Fedora to the next using yum, in particular the Upgrading Fedora using yum guide. Usually it works pretty good. I didn't really have much good experience with PreUpgrade the few times I tried it, so I wanted to give FedUp a try.
In my Parallels Fedora 17 VM it worked amazingly well. So decided to try it on my laptop, which is also running Fedora 17. I think it makes sense to do a little bit of house-keeping before running it though, and the FedUp page doesn't mention any of this (perhaps it's no longer needed?). Anyways, a few steps:
yum install rpmconf; rpmconf -a(review any .rpmnew/.rpmsave files, merge changes as required)
find /etc /var -name '?.rpm?'(find any other old .rpmnew/.rpmsave files)
yum install yum-utils; package-cleanup --leaves(review and remove any unused packages, not all will be removable)
package-cleanup --orphans(find and remove any orphan packages no longer in the repositories)
Now you can run FedUp:
yum install fedup
fedup-cli --network 18 --debuglog /root/fedupdebug.log
If this completes without error (check the log), you can reboot. At grub you'll see a "System Upgrade" entry. When that's done, it'll reboot into Fedora 18.
The wiki page talks about upgrading GRUB2 since you'll be booting from Fedora 17's GRUB2. If you've got a BIOS-based system, you can use the Updating GRUB2 configuration on BIOS systems instructions. For those using UEFI, instructions are on the same page.
You may also want to run
package-cleanup --orphans after you do the upgrade as well, just to get rid of any other leftovers. The only issue I discovered so far with the upgrade is that Google Chrome didn't work out-of-the-box. However, doing a
yum remove google-chrome-stable; yum install google-chrome got that sorted out (although it did install the unstable version; the stable version had issues with missing libraries and wouldn't load).
All in all, upgrading from Fedora 17 to 18 went a heck of a lot smoother than a fresh install did. I also got to see what the new GDM/GNOME looks like (quite nice, actually, although I think I'll give MATE a try on the laptop as well because, while GNOME3 is pretty, I definitely preferred GNOME2).
Good job, Fedora-folks! Now I just have to upgrade my main workstation, but I think I'm going to play on the laptop for a bit before taking that step. Just in case I find any other gotchya's.