Point of origin affects the rules, and both DUR-CPT and CBR-SYD are origins which allow it (but Area 1, which might be where you do all your origination, does not!). So the rules do differ even for the same fare product.

As for the reissue/revalidation/cancellation argument, I've looked up the IATA rule on this and it changed in 2005. The current rule is "Reroutings and Refunds: for totally unused tickets, there is no longer a need to refund original ticket and issue a new ticket. The ticket may be reissued based on latest fares and latest international rate of exchange (IROE) at time of reissue." I'm not sure how that differs from refund/issue a new ticket, except for not involving the credit card, as the "reissued" ticket is repriced, so it is the same price as a new issue and not necessarily the same price as the original ticket. I guess this is the essence of the situation: you lose the original ticket price (and rules) if you do change the first flight on the ticket. The new price and rules may or may not be the same. Prior to 2005 it involved a refund/payment, now it is an add collect if the price has gone up. Most people expect a reissue to be for the old price/rules and not like a new ticket, and this is simply not the case (for any IATA airline).