It's an issue when changing Red-e-deal's. The new fare must be higher than the old fare.

So if I buy BNE-SYD at $105 and then Qantas have a sale, I can't change my $105 tix to a $98 ticket if I want to change flights. I have to instead go up a class to a ticket that costs more.

The staff cannot (or are unwilling) to just change it to the cheaper ticket and charge you the standard change fee. They have to instead move you to a fare that is more than $105.

I've previously been told to just cancel a ticket, rebook a fresh flight and use the credit later rather than changing tickets due to this issue...