Comment # 15 on bug 1175967 from Dirk S.

Thanks for the explanation. I've upgraded rpm to the latest version with
--nodeps. This has broken zypper because my zypper version depended on
librpm.so.8 instead of 9. I was forced to use rpm to update zypper. After some
work caused by the dependencies I was finally able to update again.
After that I've disabled the old repository last week.
At the moment I am running openSUSE Tumbleweed 20200907-0 i586
The oldest package (except the gpg-pubkeys and openSUSE-release-appliance) is
the kernel and it has a build date of Aug 08 2020.  
Does it work too with "zypper dup" if I lower the priority of the old
repository instead of disabling?
Thanks for your help!