I pulled up that flight to see why there might have been a shift in aircraft...it looks like there was a maintenance delay out of BNA? There are a multitude of reasons why aircraft swaps occur, and I don't have an explanation for all of them.
A dispatcher would have better insight into why certain aircraft get assigned to certain flights. A DC-9 with -7 or -9 engines might not work in/out of say Midway on a certain day due to temp,winds, etc. If you had an aircraft with -9 engines out of BNA, they would swap to an aircraft with -15 or -17 engines continuing on to an airport that might have performance limits/heavy loads/long legs(heavy), etc.
A marketing rep would be able to explain why a flight might be a DC-9-30 (100 seats) on one leg and a DC-9-50 (125 seats) on the next leg.
A maintenance rep would tell you that you can stay on the aircraft you brought in and wait two hours for the generator to be replaced, or schlep down the concourse to a completely healthy jet that is up and ready.
Aircrew get to do the same dance you do....it's called the "Bag Drag" and we prefer to stay on the same jet all day long too if possible...unless I'm going into Midway...then I'll happily trade that -7 for a -17 any day.
This phenom isn't unique to the DC-9 fleet, nor Northwest for that matter. It's fairly common at most airlines that are marrying up capacity, performance, maintenance, etc. Hope I answered part of your question....thanks for flying NWA!