I thought two different actors played Trapper in the TV show
No just Wayne Rogers played Trapper John in the TV Show. When Rogers left the show after season 3, Mike Farrell was brought in to play Trapper’s replacement, BJ Honeycutt. Trapper John got enough points to return to the USA and the character went away until Pernell Roberts played Trapper John in the short lived “Trapper John, MD” TV Show years later.
The only MASH TV Show character to my knowledge to have two different actors play them was Father Mulcahey. William Christopher played Father Mulcahey in all episodes except for the pilot episode in 1972 when another actor played him.
Christopher played him every episode after, including the short lived spinoff “After Mash” starring Christopher, Harry Morgan and Jamie Farr, that aired for one season after MASH ended before getting cancelled.