The only way you can change fleet staying as an S.O is if there is a new type introduced ie A380 (intially taking s/o's off the 330) or if they reduce the numbers in the fleet ie 747 or if they flying changes for that type, eg 767 (but thats a different story
).
Apart from that they will not allow it (as far as i know, correct me if iam wrong)
Commuting on the A330 is quite easy. Perth would be a bit iffy, dont know of anyone who does that as an S.o but YBBN/YMML/YPAD all very doable.
Usually the folks going onto the A330 will start a the day before the 744.