I don't have a computer to test with right now, but I'm
pretty sure
system-upgrade doesn't put the packages in there. So at most you will be
merely copying the repo data across.
Actually, this is correct -- system-upgrade puts them in
/var/lib/dnf/system-upgrade, not in /var/cache/dnf. Copying this
folder alone isn't enough, though. One must also copy the
/var/lib/dnf/system-upgrade.json file.