Supreme Court Rule 56.01
(b) Scope of Discovery. Unless otherwise limited by order of the Court in accordance with these rules, the scope of discovery is as follows: (1) In General. Parties may obtain discovery regarding any matter, not privileged, that is relevant to the subject matter involved in the pending action, whether it relates to the claim or defense of the party seeking discovery or to the claim or defense of any other party, including…the identity and location of persons having knowledge of any discoverable matter.
(e) Supplementation of Responses.
A party is under a duty seasonably to amend a prior response to an interrogatory, request for production, or request for admission if the party learns that the response is in some material respect incomplete or incorrect and if the additional or corrective information has not otherwise been made known to the other parties during the discovery process or in writing.
Birdsong v. Christians, 50 S.W.3d 420, 422 - Mo. App. S.D. 2001 "Ordinarily, a party has the obligation to update its answers to interrogatories to advise adverse parties of any potential witnesses, thus eliminating the element of surprise at trial. [**4] " Waters v. Barbe, 812 S.W.2d 753, 758 (Mo.App. W.D. 1991). [The interrogatory in Birdsong was “Does any person other than yourself or your attorneys, have personal knowledge of the facts complained of in you [sic] petition? . . .”]
Laws v. Wellston, 435 S.W.2d 370, 374 – Mo. 1968 - Failure of a party to give his opponent the added information should be ground for refusing to allow an unnamed witness to testify, or for granting a continuance, or for a new trial, as may seem just."