Batesy, it was good to read of your happy outcome, and you have inspired me to try again. I'm in much the same boat as you.... KL-Sydney route cancelled, onward flights to India booked (and paid for) from and to KL. The difference between us is that my phone call to Jetstar elicited no help at all -- just apologies, but an insistence that, as "we are a budget airline", these things are sometimes necessary and I, as a passenger, will just have to make my own arrangements.... and wear the extra cost. Since then, I can't even get a reply to my follow-up correspondence to them.
But after reading your account, I am resolved to try again, and be as nice to them as I can. Hope I can duplicate your success, as all the alternatives I have been able to come up with so far run into many hundreds of dollars of extra expenses, which I can ill-afford (we're a family of four).
Here goes!