The "last_update_date" and "last_updated_by" fields are for audit purpose and are automatically used by the APIs.
set the parameter hr_general.g_data_migrator_mode = Y to disable entering who columns before your script.
Note: Although you can disable the ROW WHO columns from being updated as indicated, this will potentially cause an auditing nightmare if the batch update system updates incorrect data or fields. In such a case, the last person who updated the row thru the application pages or forms (i.e in a valid manner) might be accused of trickery :).
Please make sure that you test it in your test instance before moving to PROD.