Thanks, done! Wow, that was easy. My DVD drive died since installing Fedora 17, so I was hoping that there'd be such a simple upgrade process for 18.
However, I did encounter a problem: after asking for my disk password, systemd only decrypts my / partition, not my /home, which caused a cascading effect and dropped me into an emergency shell.
Running journalctl showed the original problem as:
Jan 15 18:05:45 titanic systemd-tty-ask-password-agent: open(/run/systemd/ask-password/ask.jgeHtn): Permission denied
I was very impressed with journalctl's verbose output, but it didn't tell me where "ask.jgeHtn" was created (or destroyed, since 'ls' did not find it from the emergency shell). So I had no way to get more detailed information about what was going on.
In the end, I disabled selinux (that's the fix for "Permission Denied" as root, right? ;)) and typed 'systemctl default' to continue booting, which is okay for now. Later I will see how to fix my policy properly.