When Prince William (37) and Duchess Kate (37) appeared before the altar in 2011, they were appointed Duke and Duchess of Cambridge by Queen Elizabeth II (93). This will change when William's father Prince Charles (70) is appointed king.

Prince William will then move up in succession to the throne and receives the next more prestigious title: Prince of Wales. Duchess Kate would thus become the Princess of Wales. The question is whether she wants to be officially titled that way.
When Duchess Camilla married Prince Charles, she officially became the Princess of Wales. Due to the tragic prehistory around Lady Diana, Camilla renounced the title. She is now addressed as the Duchess of Cornwall.

Catherine, Duchess of Cambridge speaks to guests as she attends a special reception with Prince William, Duke of Cambridge, hosted by the British High Commissioner Thomas Drew, at the Pakistan National Monument, during day two of their royal tour of Pakistan on October 15, 2019 in Islamabad, Pakistan.

Will Duchess Kate really become the Princess of Wales?

Imogen Lloyd Webber, the royal correspondent for the American news agency People Now, doubts that Duchess Kate wants to be known as the Princess of Wales. She may retain her title as Duchess.
Unlike Duchess Camilla, the title Princess of Wales for Duchess Kate wouldn't be a negative title bearing tragedy from the past. So it may be that she would choose this title.

Regardless of whether the 37-year-old will accept the title of Princess of Wales or not, one day Prince William and Duchess Kate will bear the titles of King and Queen of Great Britain.