Flight changed without consent or notification

I just called Jetstar about a change to my flight they made without informing me. I only found out because a friend had complained about the same thing so I thought I'd check myself. They moved my flight from late morning to late evening, meaning I would miss out on the concert I was travelling to Auckland for. Jetstar "offered" to put me on an earlier flight for a nominal charge of $120! I said I could just re-book myself online for the price of the new flight of $49 and asked, if they could move me to an earlier flight then why did they change it in the first place? The operator then threatened that if I booked online instead of with him there was no guarantee I would get that flight. He also said the fee was a "guarantee" that I would get the flight I wanted - extortion. So I asked if they are holding a seat for me right now as we speak? No, sir we are not. Then I've wasted 10 minutes talking to you whilst I could've booked that flight I saw online and I would've got it cheaper! Seriously, the WORST airline ever! Don't get sucked in by the cheap flights, it's not worth all the aggravation.