How Titles will change :
Prince William
WAS: Duke of Cambridge
NOW: Duke of Cornwall and Cambridge. He will also become Prince of Wales
The Duke of Cambridge, as heir to the throne, is now the Duke of Cornwall and Cambridge. As Charles’s eldest son, he has inherited the title the Duke of Cornwall.
William will, at some stage, be given the title the Prince of Wales - which is traditionally used for the male heir to the throne.
Since the Prince of Wales title isn't Charles's per se, but rather given to the heir apparent, he will vacate it the moment he ascends to the throne.
However, there is no automatic succession to the Prince of Wales title - the heir apparent has to be created Prince of Wales by the monarch.
The title is only 'merged in the crown' and renewed at the Sovereign's pleasure - which is why Charles became the 21st Prince of Wales not on the day of his birth, but rather aged nine in 1958.
He was only 'invested' - in the ceremony acknowledging the creation of a new Prince of Wales - 11 years later in 1969.
The same stands for the Earl of Chester.
William has also inherited the Scottish titles the Duke of Rothesay, Earl of Carrick, Baron of Renfrew, Lord of the Isles and Prince and Great Steward of Scotland.
Catherine
WAS: Duchess of Cambridge
NOW: The Duchess of Cornwall and Cambridge. She will also become Princess of Wales
The Duchess of Cambridge is now the Duchess of Cornwall and Cambridge.
When William becomes the Prince of Wales as expected, Kate will become the Princess of Wales - last used by William’s mother, Diana, Princess of Wales, when she was married to Charles.
Camilla was also technically the Princess of Wales but never used the title because of its association with Diana.
Kate will also hold the title the Countess of Chester, if William becomes the Earl of Chester.
She is also now the Duchess of Rothesay in Scotland - also previously Camilla’s title.