Sorry John for the late reply, but I just came back to it today.
The correct code was:
$('#my_select').live('change', function(){
acf.ajax.update( 'rule_name', $(this).val() ).fetch();
$(document).trigger('acf/update_field_groups');
});
I misspelled the rule name in my code.
When I got the ajax to work, I finally noticed that the matching condition was always returning false 🙂
So finally everything works, really powerful stuff these custom location rules 🙂
Thanks
Michael