Convert Contextual Filter to another FORM API Type

gistfile1.txt

/**
 * Implements hook_form_alter().
 */
function k2m_profiles_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'views_content_views_panes_content_type_edit_form') {
    if (isset($form['arguments']['field_k2m_profile_classification_target_id'])) {
      // Grab the taxonomy to make the options win.
      $vid = taxonomy_vocabulary_machine_name_load('classification')->vid;
      $terms = taxonomy_get_tree($vid);
      foreach ($terms as $term) {
        $options[$term->tid] = $term->name;
      }
      // If the value is set before, explode the string to make it an array for default.
      if (isset($form_state['conf']['arguments']['field_k2m_profile_classification_target_id'])){
        $default = explode(',', $form_state['conf']['arguments']['field_k2m_profile_classification_target_id']);
      }
      // Change the field.
      $form['arguments']['field_k2m_profile_classification_target_id']['#type'] = 'checkboxes';
      $form['arguments']['field_k2m_profile_classification_target_id']['#multiple'] = TRUE;
      $form['arguments']['field_k2m_profile_classification_target_id']['#options'] = $options;
      $form['arguments']['field_k2m_profile_classification_target_id']['#default_value'] = $default;
      array_unshift($form['#submit'], '_k2m_profiles_submit_fix_checkboxes');
    }
  }
}

/**
 * Submit handler for contextual filter.
 *
 * Problem is a checboxes passes as an array not a string, we need to format it right.
 */
function _k2m_profiles_submit_fix_checkboxes(&$form, &$form_state) {
  $flat = implode(',', $form_state['values']['arguments']['field_k2m_profile_classification_target_id']);
  $form_state['values']['arguments']['field_k2m_profile_classification_target_id'] = $flat;
}