MooTools Changing One Part Of An Object?

I'm using FancyUpload (it runs on MooTools) for a section of my website. It automatically gets the <form action=""> URL of where it should send to with

HTML Code:
url: $('form').action

I'm trying to have a drop-down list so that whenever something else is chosen, the URL of which this is submitted to, changes.I know this:

HTML Code:
<select onChange="document.forms[0].action = '/upload/?section=' + this.options[this.selectedIndex].value;">, but this, of course, doesn't change anything in the uploader.

How would I change the URL of the uploader? I really don't know how I would, since this is how the JavaScript starts:

HTML Code:
window.addEvent('domready', function() { // wait for the content

// our uploader instance [code].....