A party who has responded to a request for discovery with a response that was complete when made is under a duty to supplement his response to include information thereafter acquired in the following instances:

(a) A party is under a duty seasonably to supplement his response with respect to any question directly addressed to:

(1) The identity and locations of persons having knowledge of discoverable matters, and

(2) 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.

(b) A party who knows or later learns that his response is incorrect is under a duty seasonably to correct his response.

(c) A duty to supplement responses may be imposed by order, agreement of the parties, or at any time prior to the hearing or the submission of verified statements under the modified procedure through new requests for supplementation of prior responses.