How do you change the subdocument location in a Word 2007 master document programmatically?

Question

We have had the unenviable happen: various master documents refer to sub-documents that are no longer where they used to be due to a directory renaming. Is there a programmatic way of tweaking the HYPERLINK field without losing the master/sub-document relationship?

I've got this far ...

Sub FixyaLinks()
    Dim s 'As String
    Dim i As Long
    Dim bTrackRevFlag As Boolean
    Dim bShowRevFlag As Boolean

    bTrackRevFlag = ActiveDocument.TrackRevisions
    bShowRevFlag = ActiveDocument.ShowRevisions

    ActiveDocument.TrackRevisions = False
    ActiveDocument.ShowRevisions = False

    For i = 1 To ActiveDocument.Fields.Count
    s = ActiveDocument.Fields.Item(i).Code.Text
    If InStr(s, "CURRICULUM\\NEW") Then
        s = Replace(s, "NEW Foundation Units-in developing", "Foundation Programme Units")
        ActiveDocument.Fields.Item(i).Code.Text = s
    End If
    Next
    ActiveDocument.TrackRevisions = bTrackRevFlag
    ActiveDocument.ShowRevisions = bShowRevFlag
End Sub

It bombs on ActiveDocument.Fields.Item(i).Code.Text = s, with an error 5686 ("The operation cannot be completed because the Track Changes option in the master document does not match the option the the subdocument. Make the Track Changes option the same in the master document and subdocument.") However, I'm not entirely sure what that means.

Ideas anyone?