Pope John II who reigned from 533-535 was the first pope to change his name. His birth name was Mercurius, the name of a Roman god. That name did not seem appropriate for the leader of the Catholic Church.
"They" did not change the name of the new pope, When elected, the new pope has the choice of keeping his Christian name or choosing a regnal name that he wishes to be known as while serving. When Christ named Simon as the first pope he changed his name to Peter. After Peter, the next pope to choose a new name was Pope John II who was elected in the year 533. His given name was Mercurius and he did not think it proper for a Catholic pope to be named for a pagan Roman god - Mercury.
A pope does not have to change his name but most do. It is symbolic of Jesus changing the name of Simon to Peter at the time he named him as leader of his Church and the first pope.
Pope Benedict's first name is Joseph.
He didn't have to change his name. He could have kept his birth name but it has been the custom for well over 1000 years to take on a new name at election. Remember, Christ changed Simon's name to Peter when he appointed him as the first pope.
A person who is elected pope (Bishop of Rome) does not have to change his name, but many have done so. It is his decision, and the new name is normally announced to the cardinals when he accepts election.
At the time of Pope Valentine, popes did not change their name. He received the name Valentine from his parents.
The current pope is Pope Francis and his birth name was Jorge.
Pope Urban the 2nd
There is no rule that prohibits a pope from taking the name of Peter. However, out of respect for the first pope, no man has used that name.
First it was Pope Clement and then Pope Paul III
Since Pope Francis is the first and only pope by that name, there is no need for a number after his name. If a pope in the future also decides to take the name Francis, he will become Pope Francis II and then the original Pope Francis will become Pope Francis I.
Because his name is GERBERT!