Well, nevermind. I lucked into the solution myself.
Sometime after the upgrade attempt described above failed I refreshed the plug-ins page. This time the page displayed the expected, “There is a new version of Event Registration available. View version 6.01.07 details or update now.” I backed up everything, changed my theme to “Twenty Fourteen”, and upgraded. The upgrade was successful. I then backed up again, upgraded WordPress, changed the theme back to the usual theme used on the site, and so far all appears to be okay. I’ll know more after a little testing in this Sandbox environment. If all is well then I’ll attempt it in PROD.
For those that may also need the upgrade instructions, the following is what worked for me:
- Download the latest zip file for your plugin.
- Log into your Sandbox WordPress site.
- Back up your site’s files and DB.
- Activate a modern theme. (The reason for this is that more than once after a plug-in upgrade the legacy theme will not display the dashboard; although the site remains functional. If I change to a modern theme, make the upgrade, then change back to the legacy theme I have never had an issue.)
- Go to Plugins > Add New and click the Upload tab at the top of the page.
- Select the zip file with the new plugin version to install.
- Click “Install Now”.
- Check your dashboard to assure it works.
- Back up your site’s files and DB.
- Go back to the plug-ins page and refresh if needed.
- Select the “update now” hyperlink for Event Registration.
- Change your site’s theme back to the legacy theme you usually use.
- Test, test, and test.
- Perform the same in your Production environment if all is well.