How to override wp_insert_comment()

Question

I am trying to override wp_insert_comment() (wp-includes/comment.php). I added the following lines of code in my functions.php file, located in my theme :

add_filter('wp_insert_comment', 'my_insert_comment');
function my_insert_comment($commentdata)
{
     /* Some stuff */
}

Here is a line found in the original wp_insert_comment() function :

function wp_insert_comment($commentdata)
{
    /* Some stuff */
    do_action('wp_insert_comment', $id, $comment);
    return $id;
}

After many tests, I have the feeling that the real wp_insert_comment() is executed, and when it comes to the do_action() line, my filter is taken into consideration and my my_insert_comment() function is finally executed.

Of course, my purpose is to replace the original function by my own one. my_insert_comment() is working fine if I write it directly in the comments.php file, but I would like to keep it update-proof.

Am I doing something wrong ?