I have a flight to Italy coming up, currently booked in U, shows on Delta.com itinerary as "upgrades not applicable". I can get the same routing on a K fare, about $130 more, but my understanding is that K is also officially not upgradable. (and in looking at ticketing it on the website, it looks the same as U, "upgrades not applicable"- so what is the advantage of K?).

Any strategies, techniques, or options with either class to get a shot at an upgrade? My understanding is that neither U or K can be upgraded with miles. Do GA's have any discretion with such upgrades for GM's in those classes? Or, are they offering upgrades for a price at the gate on transatlantic flights? (and if so, how much are they asking? do they take miles, or dollars?)

Thanks in advance, and my back and bottom especially thank you (they don't like the seats in the back of the bus).