Delta requires minimum of "M" class which is expensive coach in order to use miles. I do not know for Northwest. However, Delta does not have a cheaper award for just one way travel. Like others have said, best to book directly with airlines than Orbitz. You can call Delta's international partner desk and they will check on flights available for upgrade. I believe you punch 9, 5, 2, 1 to get partner desk after dialing Delta. They would also be able to handle the Northwest flight as part of the itinerary.