Changing from one airline to another codeshare

HI All,

I am booked with AA LAX - SYD return codeshare with QAN. However i can now see that QAN has reduced the price of their premium economy seats to a point where is is only $600 more than what i originally paid for economy with AA.

AA dont offer Premium economy, so is there anyway i can change the ticket over to Qantas and pay the extra $600 to go up to Premium Economy?

Any advice appreciated

Thanks

EDIT - I am AA Plat if that makes any difference. No QAN status