Some followup here. The Terminus conversion tool is for use only if you have multi-dev. If you don’t (like me) you need to do the upstream change manually using the instructions documented at https://docs.pantheon.io/switch-drupal-recommended-upstream
A couple of notes about the manual process from my own experience:
(1) The last step, after switching upstreams, where you go to the dashboard and apply updates, didn’t work for me and I’m not sure exactly how it would work for anyone unless they have made few or no customizations of their website. Also, if the update fails, you will not know why. I see no place in the dashboard to view error details. So you may need to manually merge the upstream updates (as explained in the last part of the documentation page).
(2) Manual merge of the upstream is a little scary, or at least it was for me. There are two key changes that cause this that seem to be easily reversible:
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the merge will change the directory names for contrib code from **/composer to **/contrib. This is unnecessary. You can revert this change by editing the .gitignore file and the composer.json file
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the merge may wipe out most customizations to your composer.json file – including patches, required contrib modules, etc. I had to restore a previous version of my composer.json file and then review each individual change made by the merge to retain only those that are essential to the change of upstream.
After some trial and error (not described here) I was able to complete the upstream change. Hopefully some of this information will be helpful to others.