[Dovecot] Migarting password scheme

Hello,

by the move to Dovecot we try to alter the password encryption stored
in the database from MD5 to CRYPT-SHA256 along the Guide at
http://wiki2.dovecot.org/HowTo/ConvertPasswordSchemes. It's mostly
working but i still have not found out how to pass the cleartext
password to the re-encrypting script. According to the HowTo it should
be enough to add "'%w' AS userdb_plain_pass" to the passdb query, to
get a environment variable $PLAIN_PASS in the post-login script to
pass along. This does not work eg. PLAIN_PASS is always empty. This is Dovecot
2.0.19 from Ubuntu 12.04 LTS.

Thanks for any help

Regards

Andreas