The author name for a comment is determined by the User Name and Initials settings in the Options dialog box. (At least this was true for Word versions up to and including 2010; Word 2013 is a bit different.)
You can change the user name and initials for existing comments in the active document by using a macro:
Code:
Sub ChangeCommentAuthorName()
Dim c As Comment
For Each c In ActiveDocument.Comments
c.Initial = "MT" 'specify the initials you want
c.Author = "Miss Theorist" 'specify the name you want
Next c
End Sub
This still works in Word 2013, but Author and Initial are hidden properties in the Word 2013 object model, meaning that they will be deleted in the future.
For assistance with installing the macro, see
http://www.gmayor.com/installing_macro.htm.