Changing ticket price unsubstantiated

I recently booked a flight on the SAA website, from Kimberley to Johannesburg. This came out to R 1461.44 for two passengers. I now want to change this flight, to the next day. According to the SAA website, if I book this new ticket, it comes out to R 2239.04 for two passengers. The difference therefore would be R 777.60. If one uses the \change booking"service on the website"