The same thing happened to me earlier this year as part of a US itinerary - the initial PER-SYD sector was swapped out for a 737, however I wasn't notified, I only found out about the change when I was checking my itinerary. Pretty annoyed not to be advised, especially as the entire trip was a revenue J fare. I complained and (politely) requested consideration of compensation of either $value or points or possibly both - others had commented in previous threads that when they'd been downgraded, they received a considerable number of points as well as $compensation as well. No such luck for me - the email I received of course stated the "change of operating aircraft policy" and also that it would "not be appropriate to compensate" every time this change occurred, however they did (albeit reluctantly, from the wording) give me 10,000 points. I wasn't overly impressed as I've seen posts from pax who have received 20,000 points for things such as not getting lounge access in LAX, so I would have thought a J downgrade from A330 to 737 would be worth more than the 10K I received - however it was quite obvious that's all I was going to get!
Their reply also stated: "It may assist for future bookings to contact our agents in the Guest Contact Centre who would be happy to advise which aircraft is allocated at the time of booking" which peeved me no end, as did "Where operational requirements necessitate that the original aircraft allocated to a particular route be replaced, we will do our utmost to contact our Business Class guests to advise them of the change as soon as possible, and offer alternatives" - er, I was well aware of what aircraft was allocated at the time of booking because I booked the flight myself and deliberately chose that flight because it was the A330, something which I made quite clear in the complaint I sent, AND that I had NOT been notified of the aircraft change so they certainly didn't "do their utmost" to contact me on that occasion.