Upgrade to Business, get money back! ?
 

Maybe this makes perfect sense to the DL experts here, but it was a big surprise to me:

Today I booked a round trip from my non-hub airport to SFO, connecting both ways in ATL. It's fairly close to the departure date so I wasn't particularly surprised to see that low fare-class inventory was limited. The flights I selected had fare classes of L, L, Y, and Y (L on both flights outbound, Y for both flights returning), and the fare was about $1418. Before I committed, I noticed the box saying "upgrade for $XXX per flight." To my surprise, this time it said "Upgrade to Business class for $0.00."

Well, that sounds like a good deal, right? So I decided to see what would happen if I bit. The first surprise was that the Y fare class on the last leg actually dropped to an H fare. I expected some changes, but I would have thought that if the H fare were available initially, it would've been offered instead of the Y fare. In fact, all of the fare classes changed, from L, L, Y, Y initially to Q, I (Business Elite), P (First), and H.

And the even bigger surprise was that the upgrade was not just free, but priced out almost $100 cheaper than the all-economy fare, at $1335 or so!

If this is the result of a phenomenon that is understood here, could you explain it to me? I'd like to be able to fall into a deal like this in the future if I knew how.

Also, what is a P class fare? I'm familiar with A and I fares, but had not encountered P before. I was able to select a seat in First Class, but I'm a little concerned it could be one of those "upgraded economy" first class fares that could be downgraded if they sell out in F.

Finally, the flight that has the I (Business Elite) fare is DL15 from ATL-SFO, and is shown as a 757 with 7 rows in F. I assume this is one of the old Song aircraft with standard F seats and not the recliners that people generally associate with Business Elite, but if there's a chance I'm mistaken, that would be an even better surprise.

The easy answer is "First Class Monetization". DL has proactively been getting people to pay for first that otherwise would be given out as upgrades to elites. In some cases the charges are very small ($25) or even negative as in your case - it all depends on the fare rules and availability for your specific set of flights.

A P fare is essentially the same as an A fare. It's a coach ticket with an instant upgrade. Make sure you select a seat. If it's a 757, pick one in rows 3 or 4 to avoid problems with a last minute equipment swap to one with a different configuration with fewer seats in first. Technically if you get rerouted or have IROPS they can seat you in coach. I think it would take some extraordinary circumstances, but it can happen.

As for the BE designation, I think it has to do with the flight number originating from an international flight. Your 757 with 7 rows in F will be standard domestic service without fancy seats. Some of the domestic 75's have annoying little foot rests, though.

Apparently these UP fares are now being treated more like real first class fares. You are to be reaccommodated in FC if available. If not, you can be downgraded to Y and receive a TCV (I believe the amount was $200).

Yet I have a fare in B class tomorrow that costs the same and was not offered any of these fares and have not been upgraded..... but I am a Diamond that usually buys high fares so why would they want me to get the upgrade?

I like those footrests. One of the few nice things about nwa