Qantas have changed the aircraft on flight
QF 17 - from Sydney to Buenos Aires
Actually, it would appear they have not changed the aircraft, just changed the way they will market and operate the flight. The aircraft will still have a First Class Cabin, but they will only be operating as Business Class. This has been reported in the media in recent days and is happening on several routes for a period of several months, not just on specific days.
The aircraft change has caused the removal of the "First Class Cabin"
downgrading your seats to Business Class
Not quite accurate. This has resulted in Qantas no longer offering First Class tickets on that flight. The aircraft will still have a cabin that is the same as that used for First Class flights. But they will not be catering for First Class meals or amenities. They will be serving people seated in that section of the aircraft as business class passengers.
Unfortunately Qantas make these changes from time to time
and this cannot be predicted
Airlines do make these changes from time to time. Sometimes the changes are in fact driven by an actual aircraft change from one with a First Class cabin to one without a first class cabin. However, in this case it seems the change is one of reducing costs due to low demand for paid First Class tickets on specific routes and the removal from sale of First Class tickets for a period of several months (July to November has been reported in the media). This is not specific to your flight on that particular day, by includes several routings, including MEL-HKG and SYD-JNB as well as your flight and others.
The business class will be a flat bed
I have assigned your seats in 3A & 4 A -
individual seats behind each other.
Sounds like you will still be seated in First Class seats, which are fully flat beds, but will have business class catering and amenities for the flight.
(Is this all misinformed garbage?)
It would seem to have an interesting spin attached
Regardless of the fare paid (I assume an ACIR29SA or similar fare), you will have a Business Class boarding pass for the SYD-EZE flight and unless you have OneWorld Emerald status you will not be entitled to use the First Class lounge at Sydney before the flight.
While it is understandably disappointing, the ACIR29SA fare does not provide any financial compensation for being accommodated in business class on a flight where First Class is not offered. I feel your pain (same thing has happened to me when CX dropped F from MEL-HKG and I has routed via MEL just to fly CX F on that daytime sector), but there is little you can do about it. Your options include cancelling the ticket and asking for a refund with any penalties waived, rebooking the entire trip in business class and seeking a refund of the difference between ACIR29SA and DCIR29SA, or accepting the business class service and enjoying the first class sleeper seats on the flight.