Changing the initial flight of a ticket seems to always require a new ticket (on every airline and fare basis that I have ever seen). Often easy to work around this for a later date, but impossible for an earlier date, so you are stuck with the cancellation fees and rebooking process. Sometimes I buy tickets with an extra first leg, eg. CBR-SYD, which I fly just to activate the ticket, and then the subsequent segments are fully changable (often free, sometimes with a small fee) and good for a year. This works well when you don't know your initial travel date for certain and can afford an extra first segment.