we add a UDF UDF_STATUS_UPDATE in both SO_SalesOrderHeader and PO_PurchaseOrderHeader table to track order fulfill status. when change this order fulfill status in sales order, we want all PO generated from this sales order same UDF fulfill status get update, if sales order is brand new order, and use the generate PO button in sales order total panel to generate new PO, the value in PO get write, but if later we change UDF_STATUS_UPDATE value again in sales order and run this script in SO pre-write event, it success, but when we check the PO value, still show the value before this change

IF oSession.CompanyCode = "TEST" Then
IF oSession.StartProgram ="SO_SALESORDER_UI" THEN
IF CBool(oScript.UIObj) = "True" AND oSession.Updating = 0 Then
    'set this list to hold all po numbers
    Dim pos
    Set pos = CreateObject("System.Collections.ArrayList")

    'loop sales order detail to find all po numbers
    Set oLines = oBusObj.AsObject(oBusObj.Lines)
    retVal = oLines.MoveFirst()
    Do until oLines.EOF
        sPurchaseOrderNo=""
        retval = oLines.GetValue("PurchaseOrderNo$",sPurchaseOrderNo )
        IF CSTR(sPurchaseOrderNo) <>"" Then
                If NOT pos.Contains(sPurchaseOrderNo) Then
                     pos.Add sPurchaseOrderNo
                End If
        End if

        retval = oLines.MoveNEXT()
        Loop

        'read Sales order fulfill status
         sUDF_STATUS_UPDATE=""
         retVal =oBusObj.GetValue("UDF_STATUS_UPDATE$", sUDF_STATUS_UPDATE)

        'Update each PO fulfill status
        For Each poNo In pos
            'move to PO_PurchaseOrder_Bus
             SET oMyObj = oSession.AsObject(oSession.GetObject("PO_PurchaseOrder_Bus"))
             retVal =oMyObj.SetKeyValue("PurchaseOrderNo$", poNo)
             retVal =oMyObj.SetKey

            retVal =oMyObj.SetValue("UDF_STATUS_UPDATE$", sUDF_STATUS_UPDATE)
       Next
END IF
END IF
END IF