Sub ReplaceUM()
  Dim aFld As Field, sCode As String, sFind As String, sRep As String
  Dim sInner As String, i As Integer, i1 As Integer, i2 As Integer
  Dim sPre As String, sPost As String
  sFind = "äöü "
  sRep = "aou_"
  For Each aFld In ActiveDocument.Fields
    If aFld.Type = wdFieldIncludePicture Then
      sCode = aFld.Code.Text
      If Len(sCode) - Len(Replace(sCode, """", "")) > 1 Then
        i1 = InStr(sCode, """")
        sInner = Mid(sCode, i1 + 1)
        i2 = InStr(sInner, """")
        sInner = Mid(sInner, 1, i2 - 1)
        sPre = Left(sCode, i1)
        sPost = Mid(sCode, i1 + i2)
        Debug.Print sCode, sInner, sPre, sPost
        For i = 1 To Len(sFind)
          sInner = Replace(sInner, Mid(sFind, i, 1), Mid(sRep, i, 1))
        Next i
        aFld.Code.Text = sPre & sInner & sPost
        aFld.Update
      End If
    End If
  Next aFld
End Sub