Changing actors mid performance

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I think only on television shows and in particular soap operas, is the replacement of actors so visible, especially long running shows like soap operas. I don't think it is so noticeable in theatre as the understudy come on but it is a closed performance, only if you see it every day will you notice if an understudy is doing a role, with movies if a performer were to quit midway, their scenes may be reshot by a new actor or if it is the main actor the movie is normally shelved. Television though is different in that it'd shows last longer, though normally when a actor wants to leave they are written out, if they pass away the same thing happens. It is only long running shows with intricate story lines where they cannot remove a character easily that this is more noticeable. Michael Muhney from Veronica Mars just stepped in mid story (previous actor quit) into Young and the Restless where he will play Adam a role where he is going blind, on a serious vendetta against his stepmother, and blackmailing doctors, a true soap villain, talk about hitting the floor running. What was you favourite shows which replaced actors in mid performance, continuing as the same character but different actor and of course the scene where you as a viewer see them for the first time?