Forcing passengers to cancel their flights in order to keep their money

I booked a return flight from Cape Town to Johannesburg on the 10th November for a meeting. I am hiring a car to travel from the airport to Pretoria and back. My return flight was 17:00. I also prebooked, paid and upgraded my seats to better ones. Kulula now sent me a message to say they have rescheduled my return flight to 5 hours earlier. This is impossible as I will still be in Pretoria in a meeting. They want me to land in Johannesburg, stay at the airport, recheck in and return again. They won’t allow me to reschedule my flight and they won’t allow me to cancel my flight without keeping 100% of the fees. This is borderline ********. They know I can't make the flight so they know I will have to cancel and they get to keep all my money and rebook my seats to someone else. You can’t get through to them on any telephone line. I’ve been trying for hours. The line just rings and gets cut off after a while. I want to cancel my Kulula flights so I can fly with another airline. I refuse to believe they are allowed to keep 100% of my money by rescheduling my flights to an impossible time and forcing me to cancel.