Prepaid Seats, Fiasco

My flight to Cape Town on the 15th of December was delayed by a number of hours. This in itself was understandable as it is a busy time of year and it was predictable. However, the rest of the experience, not so much. When I booked my seats, I very specifically booked AND PAID FOR, seats in your stretch zones AND an aisle seat. I am tall and broad shoulders so I hate sitting in the middle or window seats. On arrival I was handed a boarding pass with the incorrect seat number, which I promptly queried. The lady checking me in, was little to almost completely uninterested in assisting and simply said \there is nothing we can do". I was stuck with a middle seat. Now I understand completely that seats may change when plains are changed"