It is extremely unlikely that your 120 seat A319 would be subbed with one of the two 132 seat A319s.
Mostly because of the "reverse" problem. Namely that also means a 132 seat A319 was subbed with a 120 seat A319, displacing 12 passengers. And, as those 132 seat A319s normally fly to vacation type spots, there often are not a lot of options to accommodate those 12 passengers elsewhere.
I never say never ... as over the years, I have seen some odd thinking. So, if it did happen, I wouldn't ask for a voucher of the fare difference, I would ask for an upgrade voucher, round trip, from any Y fare, usable at the time of booking. Then .... visit Australia!
Just because I stopped arguing, doesn't mean I think you are right. It just means I gave up!