That's quite a big downgrade!
JsonRollback doesn't consume the data or do any validation, it just looks for patterns and transforms them into other patterns. That is why it may appear to have processed a file successfully even though the file still cannot be imported.
Technically JsonRollback supports only 3.8.xx to 3.7.94 or 3.3.xx to 2.1.27 but you can try running it twice, first with "3.7" then using the output of that, run it again with "2.1". If that doesn't work you can look in your spine.log for clues as to why it failed. If you are still having trouble, you can post or email or 3.8 JSON.
