This one's the obvious one. When the Queen dies, Charles will become king, which means that all the formalities for a queen will have their language shifted to acknowledge a male king instead. Some of this is simple pronoun shifts: "His Majesty" instead of "Her Majesty," and so on.
One notable shift will be in the national anthem. It's been "God Save the Queen" since 1952, when Queen Elizabeth II took the throne after the death of her father, King George VI. Once Charles is on the throne, the lyrics will return to the "God Save the King" format.