When logged and want to change password users click on a form button and when an URL is set in config, the form go to this URL.
When th URL have parametters theirs parametters is stipped due to the type of send of the form (GET).
URL of change password: http://mydomain.com/index.php?appli=changepasswd&theme=toto
The form will redirect user to http://mydomain.com/index.php?
1- Use POST in this form perhaps work
2- Replace the form by image and <a href=..><img src=.../></a>
3- Replace the form by <a href=...>Change Password</a>