76. A lawyer must not personally act in a dispute if he knows or should know that he will be called upon as a witness.
However, he may act:
(1) if the fact of not acting is of a nature to cause serious prejudice to the client; or
(2) if his testimony only refers to:
(a) an uncontested matter;
(b) a question of form and there is no reason to believe that serious proof will be offered to contradict such testimony; or
(c) the nature or value of the professional services rendered by him to the client or, as the case may be, by another professional who engages in his activities within the same firm.