Foil
All BA public economy fares to Oz have the same penalty clause which says, basically, that it costs £50 for each transaction of change. Don't forget that it's the outbound part of your journey that governs which season the fare is in so if you change that outbound date and it falls into a different season you might have to pay more. Also assumption is that the same 'booking class' is available.
These days airlines try and make the rules 'simple' and as easy to follow as possible!
Taff