/**
 * Implements hook_entity_base_field_info_alter().
 */
function {{ machine_name }}_entity_base_field_info_alter(&$fields, \Drupal\Core\Entity\EntityTypeInterface $entity_type) {
  // Alter the mymodule_text field to use a custom class.
  if ($entity_type->id() == 'node' && !empty($fields['mymodule_text'])) {
    $fields['mymodule_text']->setClass('\Drupal\anothermodule\EntityComputedText');
  }
}