Password protected document with new level of security for Office
 

I have many Excel files with Office 2003 that were protected with a password.
Now with Office 2007, when I tried to save the same file I have an
information message saying if I want to reinforce the security of the
document by converting it to an open XML format. But I don't want that
message because I still have a lot of people using Office 2003 and they won't
be transfer to Office 2007 before next year. How can I save an Excel document
without having this informational message all the time? We are talking about
a few thousands documents. Is there a property that could let me save the
Excel document with the same security as Office 2003?