Well, this didn't go as smoothly as the live USB versions, mostly due to preupgrade.
I don't have a "standard" install, you might say. This is on a laptop that's not mine and Fedora is not the primary OS. So it's very space-constrained. I don't even have the recommended 200MB (now the recommendation is a whopping 500MB) space for /boot. Personally, I think that's a ridiculous amount of room to hold a kernel and initrd (even if they are generic), but that's the default install size.
So the problem is that preupgrade downloads everything to /boot, but I don't have enough room for it. The other problem is that preupgrade didn't bother to tell me either. There are some bug reports for it already, but I'm pretty sure it worked for F11->F12, so it's an annoying regression.
Fortunately, that preupgrade page has instructions for setting up the correct grub command line. I had enough room for the kernel and initrd, but not the install image. I'm not sure if it's because the downloader failed to notice it ran out of space, but the installer wouldn't auto-download the install image. I had to set a URL source, find a mirror, and point it to <mirror>/releases/13/Fedora/i386/os/.
So I've got the installer image set up and everything's good to go, right? Well, no. In the end, my space-constrained root didn't have enough room to upgrade. Oh well, might as well see how nice a full install looks. I still remember what I had installed well enough to tweak it after.
How did that go? It was pretty nice and simple, actually. The only problem I ran into was that it didn't figure out my boot loader configuration. I don't know why that is, but replacing it seemed to have worked. The only hiccup was I didn't remember which partition should have been Windows. It would have been nice if it had shown partition labels.
Anyway, after dropping stuff I didn't need (who needs all graphics drivers on a laptop?), and installing all the stuff I could remember (like devel packages), I ended up with 1.2G free. I even have OpenOffice installed this time. I don't know how I managed to go from ~100M to 1.2G, but that's definitely a great improvement.