He made the change astoundingly fast, smooth, and natural. No amount of attention paid into that video can catch how it's done.
The man in the video is Carl Rheuban. Aside from his real estate consultancy and entrepreneurial endeavors, he has been performing tricks with bills and cards before big and small crowds for almost 60 years.
The secret---the quickness of hand, that is---is not really clandestine. Such talent has been known, discussed, and revealed for centuries, but not everyone is bound to possess the ability to deceive even the keenest eye with swift and suave maneuvers.
Part of "The Secrets of Stage Conjuring" by the great illusionist Jean Eugène Robert-Houdin includes one of the earliest writings on the sly power of the hand, which he so artfully described: "My hands are short ... but that very shortness is a virtue, if not a beauty. The dexterity of the fingers is in inverse proportion to their length."
The skill relies not solely on the physiology of the hand, of course. As succinctly told in a 1912 book titled "Our Magic," the success of a magical act hinges on the workings of the mind: The real art in performance magic is largely intellectual. It requires a great deal of mental discipline to achieve precision and technical adeptness. Rheuban says he has done the trick many times. But because of the profound skill that goes into it, it never fails to elicit spectacular reactions.
Whenever he is not busy managing his businesses, Carl Rheuban performs magic tricks before intimate and big crowds. Visit this website to know more about his skills.