Must someone who has embraced Islam from Christianity, and whose name was John, change his name?

A person who converts to Islam is not required to change their name unless the name is forbidden (like 'Abd al-Masih - servant of the Messiah). If the name is not forbidden, they are not obliged to change it. Ibn Baz held the view that if the name is not good, or if it signifies servitude to other than Allah, then it must be changed to an Islamic name (like 'Abdullah - servant of Allah). However, if the name is not forbidden but is associated with disbelievers, it is preferable and better to change it.