I'd try a bit of VBA code as follows. See Graham Mayor's page for how to
install it:
http://www.gmayor.com/installing_macro.htm
I think this will do:
Sub UpdateAllFields()
Dim objRange As Word.Range
Dim objField As Word.Field
For Each objRange In ActiveDocument.StoryRanges
Call ProcessFields(objRange)
While Not (objRange.NextStoryRange Is Nothing)
Set objRange = objRange.NextStoryRange
For Each objField In objRange.Fields
Call ProcessFields(objRange)
Next
Wend
Next
Set objRange = Nothing
End Sub
Sub ProcessFields(objRange As Range)
Dim objField As Word.Field
For Each objField In objRange.Fields
' This has to be an exact match
If UCase(Trim(objField.Code.Text)) = "PAGE \#FILENAME" Then
objField.Code.Text = " FILENAME "
objField.Update
End If
Next
End Sub