Is there any term for a person who has changed his name? For example, there was a man who was named "Jacob" when he was born, but when he reached the age of 18 he decided to change his name to "Joseph". Is there any term for this kind of person? Or at least for this kind of practice?
The term is "renamed" and the practice is called "renaming" as we read in legal documents.
Well, when such changes aren't totally unexpected, (eg: a woman changing her last name upon marriage), the old name is often appended to the new one with a "nee" in the middle. Supposedly this is French for "born as", so it could reasonably be applied in any situation where a person's birth name has been changed.