The function changeOrder don't return any value in lib/Api/MarketplacesOrdersOrderApi.php.
It can be helpful to get the returning state.
Line 110 should have a "return" before calling the function changeOrderWithHttpInfo:
return $this->changeOrderWithHttpInfo($marketplace_technical_code, $account_id, $beez_up_order_id, $change_order_type, $user_name, $if_match, $test_mode, $request);