The workaround is described in the first posting and consists of converting the rpm database in a 2nd shell, when the upgrade hangs and continue the upgrade after data base conversion
Any further ones? Probably yes, I guess. TIA for completing!
A few things that are very personal, but that I could have done before the upgrade:
sudo was not installed, but it was installed during upgrade, thus I should have locked it before;
the same for openSUSE-repos-Leap, but I guess that somebody who wants to use the migration tool has it installed and wants to keep/upgrade it.
Another thing that surprised me that I did not run a firewall, but the firewall was activated during upgrade. No idea how to prevent that by doing something before the upgrade.