Well I can state this from an experience that was more of a recovery/rebuild. Backup the /etc, /home, and /var. Plus grab anything like your custom RPMs for anything that is mission critical that isnt Red Hat sanctioned (like Apache, Sendmail, X).
I had a cracked box that was more than likely rootkitted that had to be rebuilt, that took about an extra 45 minutes or so after the install, repartition (the guy that did it was an asshole, had it all as one giant partition).
Other than that, I would expect a pretty smooth sailing. Upgrades I have done usually were reformats for none other than cleanliness of the install. Plus head on over to the mailing lists. Maybe a success/failure story can be relevant to your hardware or specific software (it hated upgrading RealTek NIC's or APache needed to be toyed with or whatnot).