Probably switching due to the retirement of the 76Q.

In many markets, there are nonrefundable P fares and more expensive refundable P fares.

There is no such thing as a published P fare there is P inventory. The fares that pull from the P inventory are economy based fares with an adder that represents an instant upgrade. Be careful not to confuse the fare basis with the inventory bucket. P is an inventory bucket but not a fare basis.

Here is an example: MSP-LAX has a published E based fare that requires both E inventory and P inventory and books into the F cabin. The fare code is E0NA0FB. Note this isn't a P fare its an E fare with an instant upgrade.

The problem people run into is that sometimes agents see the fare code and assume that, since it does not start with F, C, or J that it's not a front cabin fare and put the passenger in the back.

this has been outdated for a while, the current info can be found here:

http://www.delta.com/content/www/en_...ith-delta.html

Only partially correct. There is no such thing as a P fare, that is true. However if you look at the fares that book into P, they state that they are applicable for first class and make no mention of an instant upgrade.

The fare designation construction indicates a first class fare. The first letter is the fare 'bucket' and 2nd from the end is cabin. In your example:
E0NA0FB
E = Bucket needed (and you also need the P/A/G bucket available for whatever letter the ticket is booking into)
F = First Class Cabin

The only situations where a person in F cabin should be worried about a downgrade to economy are:

(1) IROPS, where you can accept a main cabin reroute but are entitled to "hold out" for F in situations where first available is main cabin only. (I don't believe you get compensation if you "accept" a re-route in coach because doing so is optional)

(2) equipment swap to a ship with fewer F seats (you would be entitled to compensation)

In either situation, fare basis/code does not matter. In IROPS they are generally going to put you on the first seat, coach or FC. If you don't want coach, you can wait until FC opens up. For equipment swaps, the downgrades are supposed to go by elite status regardless of fare, thus no status pax get downgraded before silver, then gold, etc.

IME buying about 50% GAP fares, I've never experienced a situation where I couldn't get where I wanted in F. I have once been auto-rebooked into coach, and I simply walked up to the agent rebooking pax from my flight that went mx and asked her to move me back into F, no questions asked and was done in 2 minutes (this was even a single cabin plane that went mx--had an A fare connecting--and they were moving me onto a direct flight).

Some options are more optional than others. ("You can stay in MSP overnight and wait for an F seat, or you can fly in back and get home tonight.") That one also somehow showed up as a /WNUP fare even though I bought it in May 2016, well after I thought DL had changed the fare basis structure....

In the E example, can one select their seats in advance? (I suppose other E restrictions don't apply since you are already in F).