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I installed Tumbleweed (under KVM, though probably not relevant).
I added the packman repo.

In Yast Software Management, I set the option "allow-vendor-change".
I then went to the repositories view, and selected the packman repo.
I then clicked on the option to switch system packages to this repo.

Many conflicts showed up, all related to vendor change, and apparently ignoring
that I had set it to allow vendor change.

I cancelled out.  Then I did
zypper dup --allow-vendor-change --from packman
at the command line.  And that work with no conflicts.

It seems to me that it should have worked in Yast.  I presume this is because
of the recent change of defaults, to not allow vendor change with dup in
Tumbleweed.  However, a repo-switch should be considered an explicit request
for vendor change, so should not trigger those conflicts.

Reproducible: Always