When a free change is not a free change

I booked 3 sale flights to Denpasar from Sydney for mid-September, and took the plus bundle for an additional cost as my plans were still a bit up in the air. The plus bundle, according to the clearly printed fare rules, allows the customer to change to another flight at no cost other than for the difference in fares if the new flight is more expensive No refund if it's cheaper - fair enough. Yesterday, I decided that we would need to leave a week earlier so I had a look on the website and discovered that the flights were still on sale - in fact $10 cheaper. Yay! So I logged into my booking and entered the new date, only to find out that the sale fare is not available to me on that day. I will have to pay around $80 more per flight. After a lengthy internet chat and phone conversation, my only explanation is that you can't change from a sale fare to a sale fare. I asked where this was written in the fare rules. after a bit of argy bargy where I was re-directed to the fare rules that I had already poured over with a fine tooth comb both before and after the booking, the call centre operator conceded that it is not written there: it is just policy. Wha????
So, if I sell my house and then remove the roof before settlement day, its OK? As long as I had something written down under my pillow in case anyone asked?
I have lodged a complaint and insisted that they record the fact that the flight I wanted was on sale for the same price (actually a bit less) at the time I attempted to book. I now inconveniently have to wait 5 days for Jetstar to consider this issue, which means that I am unable to proceed with changing the date for the next booked sector, or book the tour I have arranged with my traveling companions.
So…..I wait! If Jetstar is reading this, I will be very happy to report a successful resolution on this website if there is one. I was more than happy, buy the way, with the excellent customer service following a recent bird strike leaving Coolangatta airport. The Jetstar crew in the air and on the ground managed the situation beautifully. I am a nervous flyer and I it was all very calm and the overnight accommodation was fantastic.