Migrations from every possible old Wekan version are not implemented yet.
Here are some starting points.
https://github.com/wekan/wekan/wiki/Backup
Do backups to multiple places. And test does restore work, with restore script already made.
That way you can also edit files to fix something, and also save back to MongoDB:
Take backup before runnning this.
Using sed on Linux or Mac.
sed -i 's|FindThisText|ReplaceWithThisText|g' *
Some of the database schema can be different. If you see difference in these files, you could fix it for everybody by adding new code to migrations, so old schema is converted to new one automatically when Wekan starts.