Originally Posted by
findark
It's also worth mentioning since the sticker upgrades are almost always more expensive than a change to the premium cabin. The only counterexamples I can think of are very rare cases where the sum of the segment differentials is less than the differential of the through fare (can you route DTW-SEA-LAX, maybe that would do it?), or clerical errors where the buyup amount into Z space is actually less than the fare differential (for some reason, Delta seems to maintain these numbers separately and is really bad about keeping them in sync - the offer above on DTW-SEA is still based on a $99 input whereas the fare went to $100).
LAX is a legal routing for for DTW-SEA fares (the DTW-LAX seat upgrade offer above is part of DTW-LAX-SEA itin on a single fare). But that fact does not change pricing on DTW-LAX online seat upgrade offers. As you can see below, you can get Z fares on DTW-SEA fare with LAX routing for as low as $283 (vs. $648.48 on a DTW-LAX standalone fare). The $283 price is $10 more than $273 price on DTW-SEA non-stop above due to additional taxes (but is exact same Z fare filing).
Note that upfare pricing includes additional 7.5% Excise tax on fare difference (so base fare differential is actually $93.02 which is Delta's cut on $100 price difference). Online seat upgrade offer on non-stop DTW-SEA is close to the fare differential at $99 base, but is not actually a fare differential as there is no fare exchange involved. DL could have chosen a base amount on seat upgrades so that total with tax works out to an even dollar amount like upfares do. For whatever reason, they chose to not do that (instead all the seat upgrade offer pricing looks to have base price amount in whole dollars that ends in a 9).
