So I called the DL Reservations and warned the agent that the question might be somehow technical. Basically, I explained to her that after purchasing an initial round-trip ticket to YUL through work as part of a business trip, I paid from my own pocket and via the DL app to “upgrade” from Main Cabin to First Class.

I explained that I was booked initially in the fare classes K and X, but that after “upgrading”, my reservation shows clearly now the letters “A” and “G”, which are typical revenue fare classes for (domestic) First Class. Therefore, I wanted to know which fare class would “push” into AF-FB systems when it comes to miles crediting.

She replied unequivocally that it would be on the basis of the First Class fares, and that the moment I paid for the upgrade, it triggered a change in the fare class.

I insisted by having her confirm that when pulling the info from my e-ticket number, the fare classes that someone would see would be “A/G”, and not the original “K/X”. The reason I asked is because on the upgrade receipt I received by email, it showed the e-ticket number with the previous fare classes K/X, while the transaction associated to the payment for the fare class difference was recorded under another e-ticket like number (meaning, for DL, a 006XXXXXXXXXX). She said not to worry, because under DL’s system, once you’ve paid for the upgrade (which is not a promotional seat upgrade), the new fare is reflected on your ticket.

I have no reason not to believe her, but of course, there is also a chance she might be wrong. The only way to find out is from my actual flight on June 7. I will keep you all posted about how (much) XP I will receive.

Cheers,
-vinnyc

***********
UPDATE:
So I called another Delta agent, and this one went the extra-mile by trying to access the reservation through the Air France systems (didn’t know they could do that). However, the systems were slow tonight, so instead she put me on hold in order to speak with an AF agent. After 5min, she came back to me that although under DL SkyMiles, this would count as a fare class change, under FB apparently, this is considered as a paid seat upgrade and would therefore entitle me to Eco XP.

We are now back to square one, because I am not entirely certain that AF realizes that this is not a seat upgrade like they offer on AF, but a paid fare class difference. The critical point, in my opinion, is whether or not the new fare classes flow into the Record Locator/e-Ticket, which will dictate how AF systems will credit miles & XP.

Once again, only way to know is in a practical case, which is in less than two weeks for me, so will definitely keep you updated.

So, with a lot of nervosity initially about how my paid upgrade into First Class would post on my flight LGA-YUL, I am happy to report that my first leg (initially booked in fare class K, upgraded to fare class A), was posted to my FB account with the 15XP!!!!! See screen capture attached.

i am waiting for the return leg (initially booked in fare class X, upgraded to fare class G) to post, but I am very confident it will post the same way.

Mystery solved!

Cheers,
-vinnyc

Confirmed: the whole trip was credit with 30XP (15XP each leg). Therefore, based on this experience, when you upgrade to (domestic) First Class on Delta (via the app or on the website), as long as you are confirmed to have your upgraded fare class, Flying Blue will credit the Business Class XP (Domestic if within the U.S., or Medium Haul in my case - LGA-YUL).

Attached is the screenshot I just took.

Cheers,
-vinnyc