ASP.NET Membership API force password change

Here you are, a fully tested solution ;)

protected void LoginButton_Click(object sender, EventArgs e)
{
    /****note: UserName and Password are textbox fields****/

    if (Membership.ValidateUser(UserName.Text, Password.Text))
    {
        MembershipUser user = Membership.GetUser(UserName.Text);
        if (user == null)
        {
           FailureText.Text = "Invalid username. Please try again.";
           return;
        }
        if (user.IsLockedOut)
           user.UnlockUser();

        /* this is the interesting part for you */
        if (user.LastPasswordChangedDate == user.CreationDate) //if true, that means user never changed their password before
        {
            //TODO: add your change password logic here
        }
    }
}

In case you need help in how to change password, please let me know.

Should this post be in any help for you, please tag as answer

There is no built-in functionality.

You will need to implement it yourself. Here's an example: http://forums.asp.net/p/1273575/2414481.aspx