Last week I discovered that Thunderbird had been silently deleting calendar events. ("I extracted just the titles and got the unique set:

grep -Eho '^<[^|]+' diffs/* | sort | uniq

From there, I was able to recover any important future events that Thunderbird had deleted. I didn't bother recreating old events -- I just kept the diff files for reference.

Migration

Migrating to another calendar app was pretty straightforward. I restored a relatively recent Thunderbird backup, then ran Thunderbird and exported each calendar as an ICS file to import