Help with extending QA_AJAX class?

What I'm trying to do is extend the core class QA_AJAX so that in the 'vote' function rather than calling the function 'the_question_voting' it uses a new, slightly modified one.

In my theme's functions.php I've added the following code:

class QA_AJAX_new extends QA_AJAX {
	function vote() {
		global $_qa_votes;
		$id = $_POST['post_id'];
		$post_type = get_post_type( $id );
		if ( 'question' == $post_type )
			the_question_voting_new( $id );
		elseif ( 'answer' == $post_type )
			the_answer_voting( $id );
			die( -1 );

This is essentially exactly the same as the code in the core file, but it calls 'the_question_voting_new' instead of 'the_question_voting' (on line 8 above).

I then created my new function (again in functions.php) called 'the_question_voting_new'.

The problem is that this doesn't appear to have made any difference. When clicking on the vote button it's still using the old function 'the_question_voting'.

I'm not a developer, so I may have got completely the wrong idea about how to extend a php class, but I'm sure I've done this in the past and it's worked.

Any suggestions?