I take it that we can assume this is a hyperlink field, in which case the following macro will work (replace the paths etc as appropriate) when used as a custom process with
http://www.gmayor.com/document_batch_processes.htm which will perform the file/folder handling.
Code:
Function ReplaceLink(oDoc As Document)
On Error GoTo err_Handler
Dim oRng As Range
Dim hLink As Hyperlink
Set oDoc = ActiveDocument
For Each hLink In oDoc.Hyperlinks
If hLink.Address = "C:\Path\TargetWorkbook.xlsx" Then
With hLink
.Address = "C:\Path\Replacement.xlsx"
.TextToDisplay = "C:\Path\Replacement.xlsx"
.ScreenTip = "Click to open workbook"
.Target = "C:\Path\Replacement.xlsx"
End With
End If
Next hLink
ReplaceLink = True
lbl_Exit:
Exit Function
err_Handler:
ReplaceLink = False
Err.Clear
Resume lbl_Exit
End Function