gistfile1.txt

  $field_name = 'field_article_type';
  // Get the field info
  $original_field = field_info_field($field_name);
  $settings = $original_field['settings'];
  $settings['allowed_values'] = array(
    'articles' => 'Articles',
    'news' => 'News',
    'videos' => 'Videos',
  );
  // Create the data array that will be forced manually
  $data['entity_types'] = $original_field['entity_types'];
  $data['foreign keys'] = $original_field['foreign keys'];
  $data['indexes'] = $original_field['indexes'];
  $data['settings'] = $settings;
  $data['translatable'] = $original_field['translatable'];
  $data['storage'] = $original_field['storage'];
  $data['id'] = $original_field['id'];

  // We need to manually tamper with the DB since validations won't allow
  // type & allowed_values changes for existing fields with field_update_field()
  db_update('field_config')
    ->fields(array(
      'type' => 'list_text',
      'data' => serialize($data),
    ))
    ->condition('field_name', $field_name)
    ->execute();
  // Change the schema so it holds text instead of ints
  // For some reason db_change_field() won't accept 'text' type. Doing it manually.
  db_query('ALTER TABLE {field_data_' . $field_name . '} CHANGE ' . $field_name . '_value ' . $field_name . '_value VARCHAR(255)');
  // Update the data present in the field
  $mappings = array(
    1 => 'articles',
    2 => 'news',
    3 => 'videos',
  );
  foreach ($mappings as $mapping_index => $mapping_value) {
    // Update the field with the new field key.
    // Trying to load the nodes => updating field content => saving gives a MySQL error
    // for mismatch of integer vs. text
    db_update('field_data_' . $field_name)
      ->fields(array(
        $field_name . '_value' => $mapping_value,
      ))
      ->condition($field_name . '_value', $mapping_index)
      ->execute();
  }