If William is created a Peer, it will be announced in the morning before he proceeds to the Abbey. The tradition has been to create the royal groom a Peer before the ceremony begins so his bride legally marries a Peer of the Realm and becomes a Duchess or Countess, of course with royal rank automatically as HRH Princess of the UK.
If no announcement is made before the ceremony, that very likely means William has chosen to remain styled as "HRH Prince William of Wales" and therefore, his wife would take her style automatically as "HRH Princess William of Wales". Technically, no announcement would be necessary as the assumption of this style is a given with marriage to a Prince of the UK.
However, if The Queen had agreed to allow it, I can see an announcement coming after the ceremony from the Palace stating that while Catherine was automatically "HRH Princess William of Wales", she would be permitted the use of "HRH Princess Catherine" as a courtesy.