Question about mileage upgrades
 

I'm looking at booking a trip overseas (on NWA) and I'd like to fly business but I'm also trying to be cost conscious. Looking through the rules, it seems like I should be able to upgrade anything that's booked in 'B' or 'Y'. So I do a search for the flight with the minimum fare clase off 'B' and it comes back around $2800. I take that to mean I can book the flights for $2800, upgrade each of the segments and be on my way.

But if I instead check the box for 'mileage upgrade', which presumably checks the same fare class, the price jumps up to $4500.

What's going on?

Did you try calling your WorldPerks Elite line and ask them?

(I just went through this same exercise, until I discovered there were no B fares to Tanzania and Y was something like $7,500. Gulp. So it's steerage class for moi :( )