Unable to update guid of image posts using $wpdb->update

Hello, everyone. I’m a WP developer and I’m stuck with a problem I hope you might be able to help me with.

I have a new client who has been using WP for his travel blog for around 2 years or so, with the Media setting “Organize my uploads into month- and year-based folders” unchecked, and has asked me to fix this retroactively for all images.

For now, I have successfully:
1. Copied all the images into year\month folders using filemtime() and date() to get the year and the month. No pb there.
2. Updated all references to said images inside the post_content column, in the wp_posts table.

But I’m failing at updating each image post’s guid using $wpdb->update(). Here’s the script I’m running in order to accomplish this:

function sep($str) {
    return str_replace(array("/", "\\"), SEP, $str);
}
function update_db_guids() {
    global $wpdb;
    //$db = @file_get_contents(OLDDB);
    $query_images_args = array(
        'post_type' => 'attachment', 'post_mime_type' =>'image', 'post_status' => 'inherit', 'posts_per_page' => -1,
    );
    $query_images = new WP_Query( $query_images_args );
    foreach ( $query_images->posts as $image) {
        $ID = $image->ID;
        $url = wp_get_attachment_url( $ID );
        $url_r = explode("uploads/", $url);
        if( strpos($url_r[1], "/") === false ) {
            $year = date('Y', filemtime(PATH.$url_r[1]));
            $month = date('m', filemtime(PATH.$url_r[1]));
            $new_url = $url_r[0].$year."/".$month."/".$url_r[1];
            $wpdb->update (
                'wp_posts', // table
                array( 'guid' => $new_url ), // data
                array( 'ID' => $ID ), // where
                array( '%s' ), // data format
                array( '%d' ) // where format
            );
        }
    }
}
set_time_limit(360);
define("SEP", DIRECTORY_SEPARATOR);
define( "PATH", sep(WP_CONTENT_DIR."/uploads/") );
update_db_guids();

However, after running this script and checking the targeted image posts, their guids remain unchanged.

Can anyone shed some light on what I may be doing wrong? Thanks in advanced!