Starting at version 6.0 passwords are not any more stored using reversible encryption. A MD5 fingerprint is used. So when updating from previous version, you must decrypt existing password are rewrite them in the database as MD5 fingerprint.
This operation is done using :
It is not reversible that's why you should save the database before and that's also the reason why Sympa author's did not include this operation in the automatic upgrade process.