You can only upgrade using points to Premium Econ if you have booked a freedom fare, or you can go from Premium Econ to Business using points as well. Did you book premium economy?
I was hoping to do the same on my upcoming LAX-MEL as well, but work usually books Getaway or Elevate fares, and in this instance it was Elevate, so I can't upgrade to premium economy using points which is a bit disappointing, as I have over 150k points and this is my first long haul flight. Though I guess it is a popular route, so they can't open up the flood gates too much.
With that said, I do have YX seats, so hopefully it isn't all bad. I'm about 6"4 and can't imagine I've have a good time in Y.