Appalling service

On 1 June I flew from OR Thambo to Cape Town International Airport on the 19h30 flight. I had pre-booked and paid for a seat and was allocated seat 4F. I have always pre-booked my seat as I suffer from claustrophobia which is exacerbated but sitting in the back of the plane. On checking in the very friendly staff member told me I had been upgraded (which I did not ask for). At the boarding gate I was informed by a very rude staff member that I had been down graded to seat 23F. When I tried to explain to him that I had pre-paid for my seat, his reply was "you were given a complimentary upgrade, someone has paid for that seat and you have been down graded end of story". Is this how you treat you economy class passengers. It was the worst flight of my of my life and I will never ever recommend Lift Airlines to anyone. This complaint has already been sent to Lift Airlines 2 weeks ago with zero response. The arrogance of the lack of competition