Ajax has nothing to do with JSP includes whatsoever. All of the JSP actions take place on the server in order to format the HTML page to send to the browser. The fact that a JSP (or anything else) created the HTML is completely moot.
Whatever you want to do to the page once it's delivered to the browser has nothing to do with how the page was generated on the server.
With that in mind, care to ask your question again? Because I'm not sure what you are really asking.
bill petrin wrote:What I would like to accomplish is switch the jsp include file when a user changes a drop down menu.
Apparently you didn't understand my post. Once again, there is no concept of a JSP include, or anything to do with JSP, once the response leaves the server, so the whole question of "replacing the include file" is moot. Can't be done. Period.
Now, as Eric pointed out, you can replace the DOM elements that resulted from the include, but you cannot, I repeat cannot, interact with the include itself in any way.
With a library such as jQuery, it's easy to instrument the dropdown to fire off an Ajax call to fetch new content to insert into the DOM, or to replace elements in the DOM.
But if you keep focusing on the JSP include, you will get nowhere.