I have a question then:

I have a RTW 4 cont ticket:

Already taken -
HKG-LHR-(other inter-european trips)-US

To be taken -
US-SYD-(multiple Australian domestic trips)-HKG.

Now, for the trips to be taken, can I do:
US-HKG(no stop)-SYD-(domestic Australian flights)-HKG?

I want to fly CX from US-HKG-SYD instead of QF on US-SYD. More miles and better service! Can I do that? I suppose I have to have the ticket re-issued?

Btw, if you are traveling on the same route but on a different 1w airlines, you DON'T need to re-issue the ticket!