A close friend of mine contacted CO about the same topic. Their explanation to him was that along the lines of Kosher meals for US flights are prepared only in NYC, order several days in advance, and are sent to the departing airport sometimes a day ahead of time. Because of the leadtime, the airline cannot order a FC (U) (<--- that's O-U symbol for kosher) meal when a passanger is upgraded by EUA only a couple days before the flight.
When my friend was upgraded 6 days ahead of a flight on a Y-fare, he received a FC (U) meal. Otherwise, he typically gets Coach (U) meals when upgraded.
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On the road, In the air,
I enjoy travel, From here to there.