Complex sentence: Wayne Rogers was told when he accepted the role of Trapper for the TV series that Trapper and Hawkeye would be equally important, almost interchangeable, but that changed radically when Alan Alda was cast as Hawkeye.
Split into simpler sentences: Wayne Rogers was told when he accepted the role of Trapper for the TV series that Trapper and Hawkeye would be equally important, almost interchangeable. However, that changed radically when Alan Alda was cast as Hawkeye.