<s:Body xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">

<s:Body>....</s:Body>

public object BeforeSendRequest(ref System.ServiceModel.Channels.Message request, System.ServiceModel.IClientChannel channel)
        {
           ...
                request = ModifyMessage(request);
           ...
        }

private Message ModifyMessage(Message oldMessage)
        {
            Message newMessage = null;
            MessageBuffer msgbuf = oldMessage.CreateBufferedCopy(int.MaxValue);
            Message tmpMessage = msgbuf.CreateMessage();
            XElement xd = tmpMessage.GetBody<XElement>();
            var ms = new MemoryStream();
            var xw = XmlWriter.Create(ms);
            xd.Save(xw);
            xw.Flush();
            xw.Close();
            ms.Position = 0;
            XmlReader xr = XmlReader.Create(ms);
            newMessage = Message.CreateMessage(tmpMessage.Version, null, xr);
            newMessage.Headers.CopyHeadersFrom(tmpMessage);
            newMessage.Properties.CopyProperties(tmpMessage.Properties);
            return newMessage;
        }