Rewrite comment form post ID on submit

This bit of code will update the comment_post_ID with the value submitted using your custom form.

function wpse405448_update_comment( $comment_ID, $comment_approved, $commentdata ) {
    if ( isset($_POST['acf']['field_6278d64aecf19']) && !empty($_POST['acf']['field_6278d64aecf19']) ) {
        $comment_post_id = intval($_POST['acf']['field_6278d64aecf19']);
        
        $com_arr = array(
            'comment_ID' => $comment_ID,
            'comment_post_ID' => $comment_post_id,
        );
        
        wp_update_comment($com_arr);
    }
}
add_action( 'comment_post', 'wpse405448_update_comment', 10, 3 );

Make sure both the comment_ID and comment_post_ID are valid. Otherwise it wont get updated.