Re: Replace duplicate actors.....
For about 40 years, I've thought there were two actors who should have been on TOS because they were actively working during the 60s, but weren't on the show. The first was the aforementioned Leslie Nielsen, who was primarily doing villains in those days, and would have been perfect as Ronald Tracey or any other highly respected Starfleet officer gone mad. My second what-if was Ed Asner, who would have made a marvelous Klingon.
Dr. Howard, Dr. Fine, Dr. Howard: For duty and humanity! --Men in Black (1934)