So I received a very nice reply saying that the flights had been booked and asking me to call to pay the change fee. Which I did. Ende gut, alles gut.

The helpful email said that the charges were less than they might be as the APD for a flight of less than nine hours was less than that for flights of longer. I didn't know that.

I may be moved to send a polite email thanking them for their help but pointing out that I got bad advice when I called to try to make the same changes on the phone. After all, some might have been put off and paid more than they needed to.