During V1/VA1.5 days, (pre voluntary admin, during which/where they nearly went bankrupt), they had the wide body, and their 737s were all the -800 variety, with a J cabin.
Now, they are really a LCC, but won't admit it.
Not only on the OP's flight, but also all flights or the majority of flights from CBR - SYD, are now with Link airlines, no J cabin, but they dare to charge a J fare, if you are flying further out from SYD, ie, CBR - SYD - PER, even tho the CBR - SYD has no J, they will still charge you as if there was a J cabin.
OP, as you didn't book J, and now that they have changed to plane to all Y, you don't have a choice of getting a J seat on that flight.
EconX might be an option, either row 1 or row 13/14 over wing exit row.
In a way, lucky you bought a flexY, so can get a refund and go via MEL, if you cancel and rebook.