A system upgrade reinstalls every package, but does not regenerate host keys (Fedora).
A full reinstall is wiping the drive completely, and running the installer from a LiveCD/LiveUSB. Nothing is retained, and new host keys are generated.
If my host keys were changing regularly, I would worry about it. There's no legitimate reason for that to be happening, since I'm not regularly wiping the drive and reinstalling, nor am I regularly manually deleting the host keys (the other way they get regenerated).
If my host keys were changing regularly, I would worry about it. There's no legitimate reason for that to be happening, since I'm not regularly wiping the drive and reinstalling, nor am I regularly manually deleting the host keys (the other way they get regenerated).