What I'm going to suggest is a bit drastic so don't try this first - somebody might have a better solution. If you've decided to get rid of TB, this would be worth a go.

You could save your Calendar's data files (the ones in the new version) using the export option. Also save any other data files that you use (you didn't say that you use Thunderbird as an email program).

Then uninstall TB - AND remove the profiles and data files.

Reinstall TB and then import the exported Calendar files. The reason I say this is it could be an incompatability issue with the upgrade from 115 to 128.

Before you begin, I would also save the ENTIRE profile directory as if you're moving it to a different laptop (https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data). I run Windows so I can't tell you how a Mac will store the profiles and where they are.

A second, more drastic option as a last resort, would be to follow roughly the procedure above but physically move the data files from the calendar profile into the new install.

Again, don't do this as a first option - only if you're going to get rid of the program. And only do it if you're competent with the procedure.

Thanks for responding. Actually, my partner was able to pull last week's Thunderbird profile from my Time Machine backup (I couldn't locate it). We needed to make it into a zip file and put it in under Library/Thunderbird>Profiles and pull the old profile out on my desktop and open the zip file for the profile dated last week. Then, I needed to go into Help>Troubleshoot Information. Then click on the profiles link and select that profile. It worked! I have last week's calendar and I turned off auto updates, so I should be good to go for awhile.