Using act_as_authenticated and convert existing user table



I am a rails newbie and I can’t find out how to use act_as_authenticated
an existing user table.

The problem is that I can not figure out how
to encrypt all existing passwords/logins to the
fields encrypted_password/salt for each row in my user table.

The user table is already migrated and the AAA plugin works well with
new records/users but I also need to convert the remain 28000 records.

Anybody out there who can point me in the right direction?

Thank you