According to what was published in Prince Harry’s memoirs, this was King Charles III‘s daring request to Kate Middleton to align with the royal standards after her union with the heir to the throne, Prince William.
In the book ‘Spare‘ it is revealed that the current monarch and his wife made a strict suggestion to the now Princess of Wales, ensuring that the most ideal would be that she would change the initials of her name and replace the letter ‘C’ by the ‘K’ to facilitate the royal protocols.
Harry explained in his memoir, “There were already two royal cyphers with a C and a crown above: Charles and Camilla. It would be too confusing to have another. They suggested making it Katherine with a ‘K’“. For the Duke of Sussex, this did not make sense, so he completely disagreed with his father’s request.
Apparently the Prince and Princess of Wales did not agree with the request either, as over the years Kate Middleton continued to use her name without any modification, which shows her firmness within the British monarchy, and the decision she made does not seem to have affected her relationship with King Charles III.