I thought you couldn't change the destination unless there was no change in the Avios required (e.g can change from New York to Boston but can't change from New York to Los Angeles). Wouldn't this change fall foul of that rule?

Also, I have a 2-4-1 booking SIN - SYD at the moment in WT. I've been looking virtually every day for best part of a year and I've yet to see more then 1 seat in F and one in C on any day more than about 10 days out. In fact, most days have one released at -355 in each, so as a solo traveller or as a couple happy to travel in different classes, availability is not bad. For a 2-4-1, no go. If you are a gambler, I'd book WT and try and upgrade in the last couple of weeks before you go.