Bah… I just used the recommended method in the Wiki to change Repos and then I did zypper dup. It went well, downloaded and installed 2000 files into my pre-existing, nicely working Tumbleweed. And made it unbootable. Something about an incomplete initrd or like that flashed across my eyes and it was dead and black.
It’s a good thing I created an rsync backup on 1 November. Knoppix CD facilitated restoring the rsync image nicely and all is well again.