Issues upgrading

Hi All,
I've had some issues trying to book upgrade flights and wanted to see if you could shed any light.
We want to book blights to NYC on 05/09/16 and then fly back from Miami on 19/03/16. Happy to take any flights on those days.
I checked online, and the website showed availability for Miles plus Money tickets... I thought this meant that there would be reward ticket availability.
When I called to book upgrade tickets, I was told that there was no availability on the return flight (despite there being availability based upon my understanding and the VA website) and the upgrade one way to NYC cost c.£400 in taxes (Ticket prices rose from £420 to £820 with an upgrade one way).
Has the system changed, have I misunderstood how the fare codes work, or did I just get a misinformed person at the other end of the phone?
And are upgrades always so costly re: Taxes?
I've had some issues trying to book upgrade flights and wanted to see if you could shed any light.
We want to book blights to NYC on 05/09/16 and then fly back from Miami on 19/03/16. Happy to take any flights on those days.
I checked online, and the website showed availability for Miles plus Money tickets... I thought this meant that there would be reward ticket availability.
When I called to book upgrade tickets, I was told that there was no availability on the return flight (despite there being availability based upon my understanding and the VA website) and the upgrade one way to NYC cost c.£400 in taxes (Ticket prices rose from £420 to £820 with an upgrade one way).
Has the system changed, have I misunderstood how the fare codes work, or did I just get a misinformed person at the other end of the phone?
And are upgrades always so costly re: Taxes?