Is there a reason that you skipped the motivation for the data migration in your post?
You will want to ensure you are running the latest minor version (1.25.5 or 0.11.9) of Ghost before upgrading major versions. Otherwise, you may run into database errors.
Seems like this is a strategy to ensure a stable update between major version updates. And it feels like they mixed up export from the old version and import in the new version - the page says to do the opposite…
Whatever you do, make sure to make a backup of your volume before 
A simple tar cvfz backup.tar.gz /path/to/bind-mount/folder/ (or however you do your backups) should do the trick and allows to reproduce the migration until it succeeds…