The fact that you are hit with an equipment change has nothing to do with the fact that it's still a "direct" flight. Direct flight only means that a particular flight has the same flight number from destination a to z (with potentially many scheduled stops in between -- e.g. a milk run SEA-BOI-BZN-FAR-MSP).

NW and most other carriers are notorious for direct flights that still require equipment changes. You won't know until you get to the connection city whether you are changing equipment.

The "direct" flight problem comes in when you talk about mileage & segment posting.

-RM

UPDATE: 2 flights become 1 become 2?
 

I called WP service center and got nowhere with agent so asked to speak with supervisor. I tried to explain situation (that I'd never have booked that flight if I'd know it'd become direct flight and needed 30th segment) and was getting nowhere -- lots of talk about NW not having to do anything because switch was under an hour. And I'm like, but, but, but, but ... I remember him eventually saying something like "work with me here" and finally I mentioned discrepancy in taxes and refund -- at which he suddenly said I had 2 flights.

OK, maybe I do. How do I know? I didn't get anything via email about change. Is there any way to tell from looking at flight info at nwa.com. I'm not particularly worried about not eventually getting 2nd segment (by writing to TTU afterward if it posts as 1), but I kind of hate the idea of losing upgrade potential given all the hassle.

same thing happened to me LAS-LGA
 

I had the exact same thing happen to me... I have a flight from LAS to LGA through MSP. When I booked, it had 2 different flight numbers. Now both segments have the same flight number. I called the plat line and they said that it still shows as 2 segments, and on nwa.com it looks like 2 segments as well. I even have 2 different seats for the segments! So, I think if you can pick 2 different seats, then it's truly booked as 2 segements?

Haha, success. :) Sounds like he needed you to give a "valid" reason and prompted the right one for you. Perhaps this should all go in our little book of tricks. ;)

Yes, take your PNR (the 6-character confirmation code) and your last name and plug them into nwa.com on the lower right-hand box. When you pull up your reservation, look for the portion of it that is between BWI-GRR. Make sure the flight numbers are different on the BWI-DTW(?) and DTW(?)-GRR segments. If they aren't, then it will still most likely post as one segment and you should probably call again, this time citing taxes and a refund, or simply having them rebook you on flights with different flight numbers. If this happens, perhaps it's best to just ask them to route you through MSP instead since you'd already gone through this process and it didn't work the first time. There are no "direct" flights from BWI-MSP-GRR, so you should be safe if you get that.

Keep calling, keep escalating, etc. Write a TTU if it doesn't work. It should be a simple change that they should be happy to do since you need the credit for re-qualification.

So, even if the flight shows up as two segments (same flight number) with different seats, it will be treated as one for upgrade purposes?

Exactly.

I did that and it's showing 1135 bwi-dtw and 1135 dtw-grr, but it seems as if it'd have to be that way. (All I had done was have the itinerary broken into two segments; I didn't change to a different connecting flight.) Are they supposed to give me a different flight number to get the second segment even though I'll still be on 1135? I'm confused.

No. I guess it was I that was confused as I thought you switched to another set of flights.

I guess if they still share the same flight number, I'm not sure I'd expect anything to change. I don't know what effect this "breaking" into two flights will have. If you stick with the itinerary as planned, let us know how it turns out.

This happened to me on United and that is exactly how it came out - even though the changed flight numbers made it appear as a direct flight - the fact that when it was originally booked it was 2 separate flight numbers made it wind up posting as separate segments.

Hope that is what works out for you!