Ajax form in preprocess_node never triggers
You don't use the correct function for that;

function MYMODULE_form_alter(&amp;$form, &amp;form_state, $form_id) {
  if($form_id == 'selection_node_form' //and same condition you call your function){
    //do your changes here
  }
}


Or use drupal_retrieve_form then you can override the form.
The drupal_get_form call the function drupal_build_form which build the form, you can't override it that way.