Another idea is that you could install webmin / usermin on the FTP server and have the users change their passwords that way via the web browser.http://www.webmin.com/usermin.html
Usermin is a web-based interface for webmail, password changing, mail filters, fetchmail and much more. It is designed for use by regular non-root users on a Unix system, and limits them to tasks that they would be able to perform if logged in via SSH or at the console. See the standard modules page for a list of all the functions built into Usermin.