I've changed the return on a single S class ticket multiple times without any fee, sometimes as much as 4 or 5 times. But as mentioned earlier I'm guessing it was waived due to my status.

Just be aware that you can only change the ticket if the new date you want has S class availability. If there's no S class availability, even though there's other economy class availability, you'll end up paying more. This is not unique to OZ of course....all airlines will charge you in this case, or in some cases even make you buy a new ticket. S class is a very low bucket and is one of the first to go when a flight starts filling up, or is expected to be full. So it could very well be an issue if you're traveling during peak times, or try to change your ticket at the last moment.