Ha? I don't get what you are saying.....

You stated: "I was initially booked on QF6 on PE but QF6 was cancelled on that day and QF rescheduled me to an earlier flight on QF82 and since there's no PE on A333, I was upgraded to J."

So the flight was cancelled ON THE DAY (your wording) so you actually travelled in J, did you?

If so, then what is the problem? You paid for W, you travelled in J.

Are QF trying to get more money out of you?

I would be telling them to get stuffed if this is the case, you paid for a certain cabin, they didn't provide it, you travelled in a higher cabin, but that was their decision, not yours.

Dave