Thanks, Jota. Your assumption that the existing Redmine instance is running on old hardware running an older version of Mac OS is correct. Assuming I can get a local successful upgrade of Redmine (I am attempting the upgrade process on a machine running MacOS 10.13), I will either move this to a cloud server (ideally) or newer hardware.
I was not able to upgrade successfully using the installer that you provided but it did allow me better insight into the error messages regarding existing database tables when running the migrate script. All of these tables generated "table already exists" errors: comments, user_preferences, wikis, wiki_pages, wiki_contents, wiki_contents, wiki_contents_versions, wiki_contents, time_entries, changesets, changes, watchers, changesets_issues, issue_relations, boards, messages, wiki_redirects, enabled_modules, projects_trackers. Dropping each of these tables before running migrate did at least allow the migrate script to complete.
That said, we really only use the issues portion of Redmine so all of those wiki tables aren't that relevant. What strikes me about the database after running the migrate script is that the important tables -- projects, issues, comments, etc. -- have zero records in them. Those tables do have records in them after performing a restore but prior to running the migrate script.
Do you have any other suggestions on how I can proceed?