Change Default Custom Fields Metabox Name

You need to declare the $wp_meta_boxes array as global:

global $wp_meta_boxes; 

If it still doesn’t work try:

add_filter('add_meta_boxes', 'change_meta_box_titles');
function change_meta_box_titles() {
    global $wp_meta_boxes;
    echo '<pre>';
    print_r($wp_meta_boxes);
    echo '</pre>';
}

to see what’s going on (and to check where the title is). You should also prefix your function names to prevent a clash with WP or other plug-ins.