Here are my notes on FC3 to FC5 upgrade I just completed. Had a few issues. Maybe my fixes will benefit others. Hardware is a Dell CPXJ750GT laptop (750MHz PIII).
Backed up FC3 install using TrueImage. Downloaded the FC5 DVD iso, ran sha1sum, booted from the DVD and ran media test before install. Selected upgrade and chose to stay with Grub. Don't know how long it took the install to run. After starting the install I crushed my finger working on my car and had to make a trip to the emergency room.
On first boot FC5 complained about missing icons and missing default theme. Firefox won't open at all, nor will Thunderbird. Software Updater, Yum and YumEx all fail with error about invalid baseurl. Konqueror opens (I have both Gnome and KDE installed), but can only see local files, nothing on the web. Check ifconfig looks ok. Valid IP address and all. Tried reboot doesn't help. Network hardware is Netgear PCMCIA recognized as Linsys 21x4x DEC-Tulip compatible.
Fixed the web access issue by going to System, Admin, Network and deleting network card, rebooting, and adding it back. Now Konqueror can see the web. Firefox still won't even start. Opening Firefox in a terminal offers no clues.
Fixed the default theme error on boot by doing System, Admin, Login Screen and picking a theme (none were selected). This also fixes strange issue with three dots (...) in username field after boot.
Fixed icon errors on boot by removing applications I had manually added to the top panel. I had used icons in FC3 that don't exist in FC5. Discovered I can just drag icons from the menu to the top panel and they use their own icon.
Fixed Thunderbird issue by choosing it from Applications, Internet (I was starting from a desktop icon). This ran an upgrade and now Thunderbird works OK.
Fixed Yum issue by removing two files in /etc/yum.repos.d that had "None" for baseurl. I didn't save the file names, but they started with "KDE_". Also an error generated by mono.repo so removed that as well. Now Yum, Software Updater, and YumEx all work. (BTW, I prefer YumEx to Software Updater. I like to see details on activity.) Ran a complete Yum update. (Took a long time.) These errors above were caused by repos that were not enabled, so I guess the old Yum ignored them and the new Yum doesn't.
At some point Firefox started working. Didn't do anything to fix it. One of the updates maybe?
Things with no issues: OpenOffice runs considerably faster than it did in FC3. Music Player and Xine work fine. Crossover made the transition just fine. Canon still camera connects to USB and downloads pics as before. CUPS sees my network printer as before.