This ought to work:

Sub AutoNew()
   Dim nDoc As Long

   For nDoc = Documents.Count To 1 Step -1
       If (Documents(nDoc).Type = wdTypeDocument) And _
           (Documents(nDoc).AttachedTemplate.Name _
              = ThisDocument) Then
           MsgBox "This document is based on an " & _
               "obsolete template and will close now."
           Documents(nDoc).Close _
               SaveChanges:=wdDoNotSaveChanges
       End If
   Next
End Sub

The For loop checks every open document, and closes any that match the
criteria in the If statement.

The check of the document type allows you to open the template itself for
editing without the macro slamming it closed immediately. (I hate when that
happens. <g>)

The other part of the If statement asks whether the attached template (the
one on which the document is based) has the same name as the template that
contains the running macro. It doesn't matter what that name is, as long as
it matches.