I have a ticket for early January from JFK-HKG booked in I class. I'd like to change to a flight one day earlier. On ExpertFlyer.com, I find I class available on the day before but when I called CX to make the change, the rep said that there is no I class on the flight I see it on, and that if I see I class it's because that's what is available for new purchases not for changes. (implying that there is different I class for new purchase vs. changes)

I was speaking with the general USA CX reservations #.

Is this possible/correct?