"sudo dnf system-upgrade download --releasever=27"
Neverthless, at the end of the command many recent files was signed not installable because of insufficient space.
"sudo dnf system-upgrade reboot"
and system rebooted twice but without upgrade. Re-launching previous dnf s*-u* download command resulted in skipping pkgs as already downloaded (but incomplete).So after manual remove of /var/cache/PackageKit/27 and repeating the process, this time all went fine.
Check free disk space, please! (Perhaps in GUI this doesn't happen!?)
Thank you all!