I'm no expert, but it sounds to me like there's no charge to change the ticket, provided the ticket doesn't need to be reissued/revalidated. If you want to change your ticket to another day that doesn't have the exact same booking class available, I imagine that will incur the charge, or if you want to extend the ticket beyond the validity of the ticket, likewise the charge.

S class usually does have the change restriction, but I've had good luck in getting mine changed all the time without charge, even extending beyond the original validity without charge. Same for K class. A couple times the phone agent has mentioned something about a charge, and then caught herself and said there was no charge. I'm guessing because I'm Diamond Plus and they waive the change fees for me, even though I don't think it's a published benefit. Or maybe they just like me.

Anyways, OZ never charges any call center fees for bookings/changes that I've ever heard of. So if there is any change fee, it would only be what's stipulated in the rules and no extras.