Depends what you mean by 'changing.' The real change occurs when you go to my computer|manage|local users & groups|users - if you change the name only on the XP user manager, you are changing an attribute of the user, not the actual username.

If I am not wrong, the username and display name are the same only the first time the user is established, after that if you use the XP fancy pants user thingy with the pictures and so on, you are changing the name of the user, not the username (does this make sense??? Holly cow, it is Saturday.... No wonder).

Just try the two places and play with those setting and you will see what I mean.