I tried the "Switch system packages to the versions in this repository" procedure. Yast, software management, view by repository, select one repository (packman for the example), then click on "Switch system packages to the versions in this repository". I get a long list of conflicts:

#### YaST2 conflicts list - generated 2018-04-03 21:20:07 ####

problem with installed package gstreamer-plugins-bad-1.12.4-lp150.3.5.x86_64

    [ ] install gstreamer-plugins-bad-1.12.4-lp150.5.1.x86_64 (with vendor change)
  openSUSE  -->  http://packman.links2linux.de

    [ ] keep obsolete gstreamer-plugins-bad-1.12.4-lp150.3.5.x86_64



problem with installed package gstreamer-plugins-bad-lang-1.12.4-lp150.3.5.noarch

    [ ] install gstreamer-plugins-bad-lang-1.12.4-lp150.5.1.noarch (with vendor change)
  openSUSE  -->  http://packman.links2linux.de

    [ ] keep obsolete gstreamer-plugins-bad-lang-1.12.4-lp150.3.5.noarch



problem with installed package gstreamer-plugins-ugly-1.12.4-lp150.4.1.x86_64

    [ ] install gstreamer-plugins-ugly-1.12.4-lp150.5.1.x86_64 (with vendor change)
  openSUSE  -->  http://packman.links2linux.de

    [ ] keep obsolete gstreamer-plugins-ugly-1.12.4-lp150.4.1.x86_64


...

And many more. What is the "problem"? It does not say. Obviously there is going to be a vendor change, I explicitly asked for the change when I clicked on "switch".

I asked on the factory mail list, and we concluded that this is a bug. The initial response was to enable vendor change setting, but this applies to all repos, not only to the active or selected one. The consensus seems to be that when one clicks on the option to "Switch system packages to the versions in this repository", the administrator intent is to do the vendor switch for this repository, it is implicit, and yast should not report that problem but switch.