Chances of an upgrade from Y to J are low at the best of times.

If there are spare seats in J then staff on duty travel have first priority, then staff and their family on non-duty travel. Then it's people in PE. Then people in Y get their chance. That assumes Qantas even chooses to fill the seats - they're often happy to fly with empty seats in J even though people have put in for upgrades. Sometimes they even do op-ups instead of points upgrades.

So assuming Qantas decides to upgrade people from Y into J - people get prioritised on the basis of both status and PCV. So being Platinum doesn't necessarily put you at the front of the pack. Solo travelers also tend to get priority, even when multiple seats are available.

That's a long way of saying that chances are low, and probably aren't going to be improved too much by altering your journey.