That’s just not correct information at all. The fare classes on the flight you want to move to are irrelevant. Platinum on a choice fare should be all good. Unfortunately not surprising you were given wrong information. The last time I called to Fly Ahead, I first got the overseas call centre who pretty clueless about it. Luckily they transferred me straight through to the Platinum line and it was sorted.
MEL-SYD this morning? The fog has cancelled/delayed a few flights, so the flights probably went under airport control only in the system to manage that. Still very frustrating either way.