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