The following procedure does not trigger when I open the workbook.
I tried both as read only and NOT read only. It works great when I step
thru it (F8). It would seem to me to be a problem in the This Workbook
Module, but I don't really know what to look for.

THIS WORKBOOK
Option Explicit
Private Sub Workbook_Open()
' IF File is Read Only, Call SaveAs_Message to 'force' New File Name
' If NOT Read Only, then exit procedure

If ThisWorkbook.ReadOnly = True _
Then Call SaveAs_Message _
Else: Exit Sub
End Sub

MODULE 1
Option Explicit
Sub SaveAs_Message()
Dim Msg As String, Title As String
Dim Config As Integer, Ans As Integer
Msg = " This is a 'READ ONLY' File"
..................... blah, blah, blah,
Msg = Msg & "Selecting CANCEL will STOP this Process and CLOSE
this file "
Title = "XXXXXXX W A R N I N G ! !"
Config = vbOKCancel + vbCritical = vbButton1
Ans = MsgBox(Msg, Config, Title)
If Ans = vbOK Then Call SaveAs_Process
End Sub

Sub SaveAs_Process()
' Bring up the Save As Dialog Box
Application.Dialogs(xlDialogSaveAs).Show
End Sub