Unless you want to publish the servlet functionality so that other clients can use it, there is no reason to convert to a Web service. If it is only a single client using it - and you are designing both the client and the servlet - then it is fine.
The benefit of Web Services is that you could publish your server side application so that a variety of clients could access it using XML/HTTP or
SOAP.
If you want to convert the application to Web Services technology, just as a learning exercise, then you can do that. Chapter 3 of my book shows how to do the kind of thing you are describing both with servlets and with the javax.xml.ws.Provider class for creating RESTful Web services.