DeForest Kelley appeared as 137-year-old Leonard McCoy in "Encounter at Farpoint", the pilot episode of Star Trek: The Next Generation, reprising his role from the original series. As he inspects the Enterprise-D, he tells Data, "You treat her like a lady ... and she'll always bring you home."
Martin Landau was originally offered the role of Spock, but declined. Later, Leonard Nimoy, who did accept the part, took over the role of the disguise-expert on "Mission: Impossible" when Landau left that show. Landau later headed his own sci-fi series, "Space: 1999" (1975).