Please forgive the ignorance, but how was Prince William ever a commoner? 3 levels, commoner, nobility, and royalty?
It is perfectly possible to be a royal
and a commoner.
According to British law,
only the Sovereign and the Peers are not commoners. That means that while Prince William (as the Duke of Cambridge - a peer) is no longer a commoner, Prince Harry (who isn't a peer as of now) is still a one. Similarly, Princess Anne, Princess Beatrice, Princess Eugenie, Lady Louise and even Viscount Severn (his title is by courtesy, not in his own right) are all commoners by law.
Just a note on Prince Charles. When he was born, because he was the first born son of the heir, did he receive the title the Duke of Cornwall at birth or once his Grandfather's death. When he got that title he then moved from commoner to noble or did that happen once he was made Prince of Wales.
Duke of Cornwall is a title that can only belong to the Heir Apparent to the Throne who is the son - never grandson or other relative - of the Monarch.
If, for example, Prince Charles were to die before ascending to the Throne, Prince William, as Queen Elizabeth's grandson, would
NOT become the Duke of Cornwall. On the other hand, if Prince Charles were childless and died before ascending to the Throne, Prince Andrew (as the new Heir Apparent and the Sovereign's son) would
automatically become the Duke of Cornwall.
When Charles was born, he was the Heir Apparent of the Heiress Presumptive; as such, he couldn't have the title. In fact, had George VI not passed Letters Patent 1948, Prince Charles wouldn't even be a Prince or a Royal Highness; as female-line grandson of the Sovereign, he could only inherit titles from his father and be titled by courtesy as Earl of Merioneth (his father's second highest title).
The moment George VI dies (and Charles became Heir Apparent to the Throne) he ceased to be a commoner and became a Peer - as Duke of Cornwall (and other automatic titles Heir Apparent assumes). He was created The Prince of Wales in
addition to the titles he already possessed; thus, from 1952 and onwards Charles was never a commoner.