Just returned after flying NW 784, LAS-MSP-LGA, one flight number but change of plane in MSP. F was totally full on the first leg and wide open on the second. I was told by the GA in MSP that it was "all or nothing" as far as upgrades were concerned. If you weren't upgraded for the first leg, you could not be for the second. She went on to state that normally an upgrade would not be allowed, but she would process it as an exception to the rule. Since this GA had told me ten minutes before that F was totally sold out (as it turned out, four people were upgraded and five seats were vacant in F), I take what she said with a grain of salt. Two questions...

1. What is NW policy on upgrading on the second leg of a connecting flight with one flight number?

2. Why would a gate agent say F was "totally sold out," when there were nine empty seats 45 minutes before flight time?