Forms Authentication Issue

 Hi,
I have an asp.net web application which uses forms authentication using a login form. New users are mailed a system generated p/w that they should change on first login. A change password form is presented after the user logs in for the first time.
The user is already authenticated when he enters the Change password page (from the login page). How can I ensure that the user is not authenticated util he correctly changes his password in the change password form?
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