Got a confirmed upgrade (at time of booking) on JFK-LHR operated by VS. It was a bit of an uphill battle getting the upgrade confirmed, though. The agent first said the upgrade wasn't available, so I had to confirm G inventory availability via ExpertFlyer. Then the agent told me that I could only upgrade from coach to premium economy (i.e. the way it is for AirFrance flights); I pushed back by quoting the T&Cs and he agreed after checking with his help desk. Finally, he misunderstood the rules and mistakenly booked the trip under VS flight numbers, meaning he had to rebook the flight as a Delta-marketed flight in order to get the upgrade to process. In all it took about an hour to get done.