Disappointed in Kulula flight

We pre-booked and paid for specific seats in row 5- close to the front due to lesser noise and movement - my husband gets motion sickness. Approximately a week before the flight I received an e-mail stating that our flight were canceled and we were moved to a flight later in the afternoon.When we boarded at the boarding gate, we were told that they were sent a British Airways plane and not a Kulula plane- thus the seating got mixed up and we had to move to row 24- right at the back of the plane.We couldn't understand why because the seating lay-out were exactly the same as the Kulula plane we were booked on.We had to endure the noise and movement. When we parked at OR Tambo, they told us not to get up, they were going to empty the plane from the front row for row. Why did they not open the back door as well to empty the plane quicker. Why did they not offer us a refund for the seats we paid for or explain why we lost our front seats.