The first "Guard" that was introduced into mixed martial arts (or "no holds barred fighting", or "vale tudo," or "cage fighting") was the Gracie family from Brazil. Specifically, Royce Gracie, winner of the first UFC events.
Since then there has been countless "changes of the guard," some obviously more significant than others.
The "Changing of the Guard" is a mostly symbolic act.
Not simply when one champion passes the torch to the next, a "guard" represents the fighters that came before them.
The Guard changes when a newer fighter comes to take their place on the mantle.
These are the 25 greatest fights that illustrate and represent a changing of the guard.