score:4
As is shown in airfare rules you pasted, changes are possible for a fee of AU$55, but there also may be a change in airfare if the fare class you originally bought your ticket in is full or not available on the new date you choose. This airfare difference can go either way.
The original fare rules may also have a period of validity, so depending on how long ago you took the outbound flight, you maybe limited on how far in the future you can plan the return flight. This can sometimes be gotten around by paying for a higher fare.
Best bet is to get in touch with the airline sooner rather than later to explore the available options and fare differences.
Likely you will find English speaking staff at their call center and at their main Seoul ticket office. China Airlines flies a lot of traffic from Australia, USA, Canada, etc to Asian destinations, so would have at least a few English speaking staff at each major Asian destination they market to these countries. And there is no real advantage to or difference between visiting their ticket office versus calling on the phone.