First, I would look into delay compensation as it sounds like an over 2 hour delay, and QF have a policy on this
(https://www.qantas.com/au/en/book-a-trip/flights/compensation-and-refunds-policy.html) Of course it depends on what the delay cause was and how they want to interpret that (see under "Delay under our control/not at home airport")
QF does have an involuntary downgrade policy of sorts, but all I could find is the agency page (https://www.qantas.com/agencyconnect/au/en/policy-and-guidelines/changes-reissues-and-refunds/involuntary-downgrade.html) - which only specifies that a customer is entitled to a "Fare Adjustment"
Elsewhere, there's been reference to QF calculating this against the full Y value of the sector in question, rather than the Y fare when the ticket was purchased, and further, fare breakdown on an international ticket like this means the pro rata value of the J seat for SYD-PER within the overall ticket may be different to hat is expected (in the good old days of paper tickets, one could see the "NUC" value of each ticketed sector, but I digress).
The point is, the affected person who was downgraded should be entitled to a refund of a pro rate amount for the SYD-PER leg that was downgraded. Howebver, if one accepts that, then asking for the ORC (Original Routing Credit, or in this case Original Class of Service Credit) probably won't be accepted since QF will suggest that since the "fare adjustment" was paid, then one has, in effect, paid for and only entitled to the Y credit for the flight taken.
I definitely think it's worth pursuing as this is a large part of the overall itin that's been affected for one pax and if the delay was caused by QF (yes, seems so) then they're on the hook for it imo.
good luck.
