Kulula don't make all their flights available if you need to change your ticket

I booked a ticket from JHB to Cape Town return. I had to later change the ticket. I went on line to see the various flight options but could not do it online. I had to phone Kulula to change the tickets. There is a standard change fee of approx R400 which I was aware of. There were a number of cheaper flights on the online flight schedule which suited me. When I phoned the Kulula office I was informed that a few of the flights - the cheaper ones were not available if you were changing the ticket. I find this unacceptable, abusive and immoral. I thought Kulula was a ethical company. Now have my doubts. In the end I just cancelled the ticket and booked on SAFAIR which was about R1500 cheaper than any of the flight options they would give me to change the ticket ( on the phone). Kulula sucks