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 :sympa.pl --md5_encode_password
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.