In an interesting British tradition, when he ascended to the throne, Charles could have selected any of his four names — Charles Philip Arthur George — as his formal king name, per the BBC. He is no longer known as the Prince of Wales, either, a title that may be bestowed to Prince William, who is now heir to the throne.

Source: https://www.distractify.com/p/what-will-prince-charles-be-called-when-he-is-king