The reserved seats will have to be released, at which point yield management may not make them available for you -- or may make them available SYD-SCL but not AKL-SCL. This is quite common on that route, so do not assume that you can change it to originate in AKL using the same seats you already have. Also there is no way for the agent to know this ahead of time, except by some high-level supervisor intervention. Probably it is much easier to just fly to SYD from wherever you are in NZ and travel as originally booked on LA, unless you have some friends in high places at LAN.