Suppose that (as has been encouraged in an earlier post) one changes a "userpqrs" name to a somewhat more informative name. To what degree does the history of userpqrs disappear? Are questions, answers given the changed name? What about comments?
Changing the name of the account doesn't change anything. All your previous answers, questions, comments get the new changed name. The history remains intact with your new username.
However, what does not change are comments and answers by others in which you are referred to by your former name. So any time somebody else has addressed you as "user6312" you will still be "user6312" in that answer or comment.
This can cause some conversations to look strange. For instance, one of the top users on the site had the name "Moron" for several months and only recently changed it. So now you can see many conversations in comments and answers to old questions in which this person is addressed as "Moron." I imagine it appears inexplicably insulting to people who don't know the history - and kind of funny to those of us who do. :)