You could probably use the custom process of
https://www.gmayor.com/document_batch_processes.htm to make the changes, but you will have to modify your macro to work with it. There will inevitably be formatting changes, but without knowing what is in the documents it is impossible to be precise about what needs to change. The following will get you started.
Code:
Sub Resize(oDoc As Document)
If oDoc.PageSetup.PaperSize = wdPaperLetter Then
oDoc.PageSetup.PaperSize = wdPaperA4
End If
End Sub