I had the reverse situation: I am PM on Delta and purchased an AM flight via the AM website (it's also sold as a Delta codeshare at Delta.com), because it was $500 cheaper on the AM site. The Aeromexico phone staff assured me I was eligible to be upgraded, but the Dulles staff had never heard of such a thing. I sat in the back to MEX. From MEX to IAD while checking in at SkyPriority without even asking the agent upgraded me to first (would you like an aisle or window, seņor?). There were 6 passengers including me in the 12 first class seats, and quite a bit of room available in back too. Nice flight, I'd definitely fly them again in either class.