This I cannot comment on. I've always installed each version from scratch, because of all the horror upgrade stories I've been reading (for any distro...) since the 90's...
linux.org.uk is running FC by update. Some of the boxes started off somewhere around RH7.2 ..
There are certainly corner cases - notably when you've got stuff from non RH repositories that the system thus can't update from CD during a CD update - something the current FC work on extras merging and single CD is going to finally sort.
Alan