Here is some VBA you can add as a macro to change all the Word documents in a given folder.
WARNING: Make a backup copy of your files prior to running this code.
Open a new Word document, paste this code into the VBA window (Alt+F11). Make necessary changes to the path, then close the window.
Dim myFile As String
Dim myPath As String
Dim myDoc As Document
'Change to the path where your documents are located.
'This code changes ALL documents in the folder.
'You may want to move only the documents you want changed to seperate folder.
myPath = "C:\temp\"
'Closes open documents before beginning
'Set the path with file name for change
myFile = Dir$(myPath & "*.docx")
Do While myFile <> ""
'Open the document and make chages
Set myDoc = Documents.Open(myPath & myFile)
myDoc.PageSetup.PaperSize = wdPaperA4
'Close and saving changes
myFile = Dir$()
msgbox "Process complete!"
Open the Macro window (Alt+F8) and choose
ChangePaperSize, then click run. The current open document will close and other documents will open and close as it makes the changes to each document in the folder.