classes

[mp3909]

So I am learning classes in python and I stumbled across something that did not make sense to me.
In the code below, when I change the name of the student via calling the method changeName passing the new parameters (forename and surname), why doesn't it reflect the changes when I print the attribute fullname?

class Student:

    def __init__(self, forename, surname):
        self.forename = forename
        self.surname = surname
        self.fullname = self.forename + " " + self.surname

    def changeName(self, forename, surname):
        self.forename = forename
        self.surname = surname

s1 = student("billy", "smith")
print(s1.fullname)
s1.changename("tom", "jones")
print(s1.fullname)              ----> why is this not updating?