Postponing a flight cost is more than the ticket - why?

Hi
Booked a return flight to NYC in December, I am flying UC and paid for the flight in cash as it was something like £1300 which I thought was great and plus you get 10 tier points so figured that was better than using my miles.
I have since considered changing it and I was told it would cost something like £4000!!, could this be right or is it worth me calling up again?
My ticket is z class.
Thanks
Booked a return flight to NYC in December, I am flying UC and paid for the flight in cash as it was something like £1300 which I thought was great and plus you get 10 tier points so figured that was better than using my miles.
I have since considered changing it and I was told it would cost something like £4000!!, could this be right or is it worth me calling up again?
My ticket is z class.
Thanks