If you downgrade from one release series to another, there may be incompatibilities in table storage formats. In this case, use mysqldump to dump your tables before downgrading. After downgrading, reload the dump file using mysql or mysqlimport to re-create your tables. For examples, see Section 10.13, “Copying MySQL Databases to Another Machine”.
A typical symptom of a downward-incompatible table format change when you downgrade is that you cannot open tables. In that case, use the following procedure:
Stop the older MySQL server that you are downgrading to.
Restart the newer MySQL server you are downgrading from.
Dump any tables that were inaccessible to the older server by using mysqldump to create a dump file.
Stop the newer MySQL server and restart the older one.
Reload the dump file into the older server. Your tables should be accessible.