enable entity_translation revision support for existing site (Non previous revisionable data)

enable_revision.php

<?php

/**
 * Enables entity_translation revisions support.
 */
function my_module_update_70XX() {
  $revisionable_entity_types = array();

  $query = db_select('entity_translation', 'et')
    ->fields('et', array('entity_type'))
    ->distinct();
  $entity_types = $query->execute()->fetchCol();

  foreach ($entity_types as $entity_type) {
    $entity_info = entity_get_info($entity_type);
    if(!empty($entity_info['entity keys']['revision'])) {
      $revisionable_entity_types[] = $entity_type;
      $query = db_select('entity_translation', 'et')
        ->fields('et', array('entity_id'))
        ->condition('et.entity_type', $entity_type)
        ->distinct();
      $entity_ids = $query->execute()->fetchCol();
      $entities = entity_load($entity_type, $entity_ids);

      // Set the current revision_id as the entity_translation revision_id.
      foreach ($entities as $entity) {
        list($id, $vid, $bundle) = entity_extract_ids($entity_type, $entity);
        db_update('entity_translation')
          ->fields(array(
            'revision_id' => $vid,
          ))
          ->condition('entity_id', $id)
          ->condition('entity_type', $entity_type)
          ->execute();
      }
    }
  }

  // Copies data from entity_translation to entity_translation_revision table. 
  if (!empty($revisionable_entity_types)) {
    $query = db_select('entity_translation', 'et')
      ->fields('et', array('entity_type', 'entity_id', 'revision_id', 'language', 'source', 'uid', 'status', 'translate', 'created', 'changed'))
      ->condition('et.entity_type', $revisionable_entity_types);
    db_insert('entity_translation_revision')
      ->from($query)
      ->execute();
  }

  // Enable revision support setting variable to TRUE.
  variable_set('entity_translation_revision_enabled', TRUE);

  return t('Entity Translation revision support is enabled');
}