No seat allocated

I booked a return flight to Bali in Aug 2013, I paid the $7 each way to pick my preferred seat. My girlfriend decided she would come with me and would book a seat on the same flight as close to me as possible. I sent her my itinerary she said those seats don't exist. I called jetstar which took me a whole day to get through to be told that my plane had been upgraded, I said great what are my new seat numbers? We haven't allocated you a seat miss jetstar said, but I can do it now . I chose my seat 6 months ago and the flight is nearly booked out now. I was in shock surely if they changed the plane they would allocate people who had picked and paid for a seat a new seat. No!!! That is a preferred seat not a guaranteed seat miss jetstar said . The more I spoke to her the more frustrated and agitated I became, I turned from a calm understanding logical lady into an outraged angry passenger. Why would they not reallocate a seat to passengers who had already booked, the plane was changed in Oct, I'm sure I wasn't the only person booked with a chosen seat, I would of arrived at the airport thinking I had great seats only to find out they didn't give me a seat when they changed my plane. The seats I had are now $25 each way to book I asked her for some customer service and just give it to me one way, as I had that seat before they changed the plane. No!!!! So I said can you take the $7 off that I paid each way for seats I don't have off the $25 no its non refundable. Arrrgh! !! I want to scream I had that seat, you took it off me but took my $7 and didn't even reallocate me a seat in replacement, she said you can put a complaint in writing if you are chosen someone will get back to you within 15 days, I fly out on 13th March.....thanks jetstar