Exact same thing happened to my in-laws returning from CNS to KIX on Monday (would have been on the same flight maybe, they got rerouted through NRT and JQ booked them on a domestic NRT-KIX flight to get them into KIX at no extra cost to them and they arrived a few hours later than planned, this is still a better option than being dumped at NRT when you have an existing hotel booking at KIX airport! Especially if you live closer to KIX than NRT) - this was only possible because Mrs eastwest101 was on the ball with initial change of flight/cancellation email, and specifically asked for that solution. I bet there were people who were not so lucky, who ended up god knows where, or got an unexpected extended layover in CNS.
Coincidently - the inlaws also lost their preferred and extra leg room seats so they will be pursuing this now that they are home in Japan. I wonder if they have any more luck dealing with Jetstar in Japan, as I would presume that they have a call centre in Japan for their Japanese customers? Please correct me if I'm wrong. My father-in-law also had travel insurance so will hear from him if JQ aren't co-operative but I would think travel insurance would have only come into play if they missed a flight, had a serious delay or ended up in NRT or something wierd like that.
I have taken the same approach with last minute seat swaps and aircraft substitutions with JQ - anything less than the seat I selected should be grounds for refund of seat selection fee, its always been a battle though as they try to fob you off with the "you got a seat story" first and then they try the we'll only refund as a gift voucher trick. As the seat selection fee is bundled into the price of the airfare upon purchase I would be interested to see if others here have had any success getting fees like this refunded - and refunded in cash too - not as JQ gift vouchers.
Mattg - what happened to you? Did you end up at NRT or did they re-route or somehow compensate you? Assuming that you brought your ticket through the JQ.au website how are you going to approach getting a refund for the seat selection fee? Am guessing you may not have time to deal with this refund issue until you return to Australia?