I think your right about the DXB-HAM-JFK jets going to perth. I beleive the switch is simply because the triple sevens are better used elsewhere. The A380 delays have meant EK can't grow as fast as it would like. It therefore must maximise the resources it has. Perth obviously can't warrant the Boeings...but perhaps benefits by the more luxuriously fitted A345s instead of perhaps A340-300s.