Last night, I completed the easiest Fedora upgrade I've ever had, without having to download and boot from a CD/DVD. Here are the instructions I followed: http://fedoraproject.org/wiki/YumUpgradeFaq
I did have some unexpected glitches, mostly due to SELinux.
- The pidgin (formerly known as gaim) system tray icon didn't appear until I deleted my .gconf settings and logged back in. This has the drawback that I had to reconfigure several aspects of my UI. It may be possible to only delete ~/.gconf/apps/panel/applets/systray (or to remove it from the panel manually), then re-add it by right-clicking the panel, selecting "Add to Panel" and chooise "Notification Area".
- The Dovecot IMAP server wasn't allowing Thunderbird to connect. I ran 'yum upgrade', and it installed an updated SELinux policy pack. After restarting, IMAP services functioned properly.
- HAL didn't start up. To fix it, I ran 'touch /.autorelabel' as root and rebooted. Since then, I've learned that using fixfiles is a safer approach.
- FreeNX didn't allow me to connect unless I disable SELinux. Later, I figured out how to configure SELinux to allow FreeNX.