Was just reading about the 15 things that happen after this queen dies:

"When the Queen passes away, Prince Charles will make a speech in the evening and will be proclaimed King at 11 AM the next morning when he swears an oath known as the accession declaration.

Heralds will read a proclamation all over the city, there will be trumpets sounding, cannons will go off in a royal salute, and the flags will fly high again. Charles’ coronation won’t happen until after the mourning period, however."

The new king choses a new name:

"If you were expecting him to become King Charles, you could be mistaken. Reigning monarchs choose whatever name they like, and it might be different than what they’ve been called all along. For example, Queen Elizabeth’s father was called Prince Albert until he became King George VI."