A party who has responded to a request for discovery with a response that was complete when made is under no duty to supplement his response to include information thereafter acquired, except as follows: A party is under duty to supplement timely his response with respect to any question directly addressed to: The identity and location of persons having knowledge of discoverable matters, and The identity of each person expected to be called as an expert witness at the hearing, the subject matter on which he is expected to testify, and the substance of his testimony; A party is under a duty to amend timely a prior response if he later obtains information upon the basis of which: He knows the response was incorrect when made; or He knows that the response, though correct when made, is no longer true and the circumstances are such that a failure to amend the response is in substance a knowing concealment; A duty to supplement responses may be imposed by order of the hearing officer or agreement of the parties.