/**
 * Implements hook_field_widget_properties_ENTITY_TYPE_alter().
 */
function {{ machine_name }}_field_widget_properties_ENTITY_TYPE_alter(&$widget, $context) {
  // Change a widget's type according to the time of day.
  $field = $context['field'];
  if ($field['field_name'] == 'field_foo') {
    $time = date('H');
    $widget['type'] = $time < 12 ? 'widget_am' : 'widget_pm';
  }
}