Got almost caught out this myself in our recent OWE.

The QANTAS reasoning is that the $25 fee is for revalidating the ticket (basically charging for the time they spend), even though changing the ticket date/time itself is free (by the OW rules).

We had one very helpful agent in the CNS office spend half an hour revalidating 6 tickets (twice!) without mentioning a charge, then a very stroppy agent in the PER office demanding the $25. In the end, we didn't bother getting them revalidated, and the agent at check-in didn't notice/care that the paper ticket was for a different date.

All in all, a bit like internal tax for flights in Chile - seems to depend on the agent you get !

Enjoy your trip !

Stewart