I would presume it doesn't work for you because the font name has been locally overridden on your instances of that style. Certainly the following works for me if there is no local font formatting.
Code:
Sub ChangeStyleDef()
ActiveDocument.Styles("Normal").Font.Name = "Times New Roman"
End Sub
If that fails on your document, does adding a font reset resolve that particular issue?
ActiveDocument.Range.Font.Reset