Presumably (since the form changes after selection) there is a javascript callback that does the changing. So you would place you analytics tracking call into that callback and read the selected value before the form changes. This requires control over the js code.
If the form is changed via an ajax call you might want to look if the setup of your page allows you to hook into the ajax success event. Jquery has global ajax event handlers, in prototype.js the same thing is called ajax responders and I'm sure other frameworks have something similar.
Another, and possibly the easiest way is to attach the onclick event to the link that changes the form and write a function that selects the value for the selected option.
Using jquery it could look like this:
$('#list1').find(":selected").value();
In plain javascript it should look like this (untested):
var list = document.getElementById("list1");
var selection = list.options[list.selectedIndex].value;
So your complete callback would look something like:
function getSelection(element) {
var list = document.getElementById(element);
var selection = list.options[list.selectedIndex].value;
_gaq.push(['_trackEvent', element , selection);
}
You'd need to attach this to the onclick event of the link and pass the id of the combobox as parameter.