Check new form-input against old form-input
When you change something in form, that change can be found in $form_state. You need to add an AJAX callback and do your changes in that callback.

Sample code:

function MYMODULE_form_alter(&amp;$form, $form_state, $form_id) {
  switch ($form_id) {
    case 'MY_FORM_ID':
      $form['field_select']['#ajax'] = array(
        'event' =&gt; 'change',
        'callback' =&gt; 'your_ajax_callback',
        'progress' =&gt; array('type' =&gt; 'none'),
      );
    break;
  }
}

/**
 * AJAX Callback.
 */
function your_ajax_callback(&amp;$form, &amp;$form_state) {
  // dpm($form_state);
  // Check $form_state here and update subscription.
}


Hope this helps!