Sorry, if she is a Princess, she is Princess William or, more correctly, her full title and style is Her Royal Highness Princess William, Duchess of Cambridge, Countess of Strathearn, Baroness Carrickfergus. And, until such time as HM Queen Elizabeth casts off this mortal coil, she will remain Princess William or Catherine, Duchess of Cambridge.
It serves no one, and irritates many when, as in this case, you are aware of her correct title and choose to "elevate" her. Those entitled to be called Princess in their own right are either born to it or, on very rare occasions, given that right, as in the case of Princess Alice.
To me it is just as rude as those who persist in calling both Catherine and Camilla by the name the held before their marriage.