I had reason to change the date of an O class ticket last week. The initial online page said the change fee would be USD90, which is what the original fare conditions said, plus any difference in fare. However, once I clicked through to the payment page, there wasn't any difference in fare for the new flight, and it only charged me USD30 as a change fee instead of USD90. If you didn't get as far as the payment page, it might be worth trying again in case the same happens for you, and the lower fee is what the telephone operator saw. You aren't committed until you actually pay.

(BTW - I'd already checked the cost of a new booking for the revised date and it was exactly the same as I had paid for the original, so I wasn't benefitting from a cheaper fare. Just some unexpected online largesse!)