I just recently finished my OWA, I had already taken 3 of 5 legs at the time of calling Qantas (2 were yet to be taken and I wanted to change Leg #4) - I was told that once the itinerary starts and you've flown some legs, that for the remaining unflown legs, only a change of dates is possible and not a change of route, even if it's to the same destination.
I called Qantas when I was in London in an attempt to change my direct BA flight from LHR to SIN in PE (Leg #4) to be via either Doha (QR) or Dubai (EK) instead.