As Prince William's wife, Kate Middleton's title as Duchess of Cambridge automatically changed when Queen Elizabeth II sadly passed away.
Following His Majesty King Charles taking the throne in the aftermath of Queen Elizabeth's death in 2022, his son Prince William became next in line for the crown and received a title change, meaning the same happened for Kate Middleton.
William and Kate's titles were updated to Prince and Princess of Wales, a title historically held by those first in line. In fact, this title was officially granted to Prince Charles at his investiture 50 years ago, and he's the 21st man to hold the title.
When Charles married Diana Spencer in 1981, she automatically became Princess of Wales. And while technically the title should have passed to Camilla Parker Bowles upon her wedding to Charles, she turned it down out of respect to Diana and was instead known as the Duchess of Cornwall (now the Queen).
However, now that Charles has ascended the throne as King, the former Duchess of Cornwall and Cambridge accepted this title in honour of Prince William's mother.