Yeah, it sounds like you've got a really basic fare type which would have very limited availability. Busy time of year to be travelling around the southern hemisphere. As Eurofan said, you might need to upgrade to a different fare type.
Alternatively, if you're travelling on a RTW ticket, it might be possible to reroute your flight (for the same $75 fee, plus any additional airport taxes). I was able to do that myself last year, rerouting the flights with stopovers in Bejing and Helsinki on the way back to Ireland.
If you're travelling on standard return tickets then unfortunately this won't be an option and I think the fare type upgrade would be your only option.