Just today I upgraded from 39 to 40 but there was an issue:
dnf told me I needed some 800k more space in my /boot partition to proceed.
I had two kernels in /boot so I dnf removed those associated with the older
of the two kernels. I then successfully upgraded.
[...]
I realize one way is to backup /home and then reinstall Fedora but
1) that seems like a lot of work and
2) it would mean that the machine in question would then have to
use Wayland rather than Xorg.
Bots have taken over the internet, and contrary to "bot" wisdom, Fedora 40
still has Xorg.
Systems upgraded over many years often have packages that not present in
current F40. Resulting issues can be difficult to understand as few users
will have those old packages. Doing a fresh install every few years keeps
you closer to a configuration others have, so any issues you do encounter
will probably affect others, but "all problems are shallow given enough eyes".
A fresh install also gives you the option of moving to xfs or btrfs.