Get order number

Hello

We want to display the order number in the E-mail subject. Therefor we use this code snipped here:

add_filter( 'woocommerce_email_subject_customer_completed_order', 'change_completed_email_subject', 1, 2 );

function change_completed_email_subject( $subject, $order ) {
global $woocommerce;

$blogname = wp_specialchars_decode(get_option('blogname'), ENT_QUOTES);

$subject = sprintf( '%s, Deine %s Bestellung wurde versendet! %s', $order->billing_first_name, $blogname, $order->get_id() );                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
return $subject;
}

Its working. But I always takes the “old” or lets say “post ID” number instead the new order from the sequential plugin.

How can we get that number?

Cheers