add_action( \'woocommerce_thankyou\', \'change_order_status\', 10, 1 );
function change_order_status( $order_id ){
     if( ! $order_id ) return;
     $order = wc_get_order( $order_id );
     if( $order->get_status() == \'processing\' )
     $order->update_status( \'pending\' );
}