M*A*S*H Ad: "Fallen Idol"

I found a couple more TV ads for the show while reading some articles on the passing of Allan Arbus. This one, for SSN 6's "Fallen Idol" is pretty accurate, but I wonder: where does "Alan Alda and Harry Morgan star" come from? As per the show's credits, Alda and Mike Farrell were the top two stars of the show, so whose decision was it to replace Farrell's name with Morgan? Not that it's inaccurate in any way, but I am curious as to how it was decided to do that.