Comment # 8 on bug 954117 from Gabriele Mohr

If you want to follow Tumbleweed you can use 'zypper dup', means do a
distribution upgrade. For an upgrade, either YaST Upgrade or 'zypper dup', its
by default allowed to change the vendor, the architecture, downgrade package
versions....
New options are added to /etc/zypp/zypp.conf which only concern the upgrade
(dupAllow...). To have the possibility to protect e.g. packages from an
external repository during an upgrade you can change the behaviour with these
options, 
see https://discourse.nordisch.org/t/living-on-tumbleweed/379.