It's not the opening of the documents that is the problem; it's the file format you're saving them in. They all need to be saved in the docx format - and you can't just change the extension to achieve that. However, since you haven't posted the relevant Save code, we can't give specific help there. That said, the code to ensure the active document is saved in the docx format might look like:
Code:
With ActiveDocument
' Save & close the output document
If UBound(Split(.FullName, ".doc")) = 0 Then
.SaveAs FileName:=Split(.FullName, ".doc")(0) & ".docx", FileFormat:=wdFormatXMLDocument, AddToRecentFiles:=False
Else
.Save
End If
.Close SaveChanges:=False
End With