woocommerce_email_subject_customer_completed_order

function change_admin_email_subject( $subject, $order ) {
    // Get items
    $items = $order->get_items();

    foreach ( $items as $item ) {
        // Get product object
        $product = wc_get_product( $item['product_id'] );

        // Get product id
        $product_id = $product->get_id();

        if ( $product_id == 153 ) {
            $subject="¡Bienvenida al Curso Online 4";
        } elseif ( $product_id == 192 ) { 
            $subject="¡Bienvenida al Curso Online d2"; 
        } else {
            $subject="¡Bienvenida al Curso Online 3"; 
        }
    }

    return $subject;
}
add_filter('woocommerce_email_subject_new_order', 'change_admin_email_subject', 10, 2);