Updating Params

<script type="text/javascript">
kUrl='http://keitaro_url/';
function update_tokens(subId, param, value) {
  var img = document.createElement("img");
  img.src= kUrl + '?_update_tokens=1&sub_id=' + subId + '&'+ param + '=' + value + '&return=img';
  img.height=0;
  img.width=0;
  document.getElementsByTagName("body")[0].appendChild(img);
}
</script> 

Now, you can call such function:

update_tokens('{subid}', 'sub_id_8', 'test')

In case, the landing page is already using Tracking Script, you can just call this method:

KTracking.update({sub_id_8: 'test'})