_ding_user_update_old_accounts generates the old hash based on the logged in user and checks if the user exists based on that hash. If the user exists her/his hash is updated in the DB with the new hash.

This can not be done in a update function as we need the users information to create the old hash and to find the user. We store no identifying information about the user, so we don't have the information required to update the hash before the user logs in.