I don't think you can downgrade at all. I think the only way to go back is to restore the last backup that you did before upgrading.

The reason being that the database design changed over time and the database schema gets updated. An older version would not be able to work with a newer schema. Another reason is that the data (process and live data such as work items and SCM) might have been converted to the needs of the newer versions requirements and formats. This is done in on- and offline migration processes.

I am also not convinced that you can use the repotools -export command to export the database to a tar file and then import that in an older version. This would be the only chance at all, but I don't think this is something that has been a design objective or tested. You could try, but I would expect this to fail with a version mismatch.