One of the reasons many movie franchises are so beloved is that they let the audience see the same actors over and over again. It gives a sense of continuity, and makes the characters feel more real as they grow and change. But there have also been many times actors were replaced in movie franchises, and regardless of whether the recasts were good choices or bad ones, the switcheroo can be a little jarring. Do producers think viewers won't notice something as major as two actors who played the same role in a franchise?
A lot of factors go into these recasts. Some actors weren't available to reprise the role, some were fired, and some simply chose not to return. Nonetheless, characters who were recast in franchises often require some time for adjustment. When you associate one actor with one specific character, another actor in that same role takes you out of the story and into your head. Recasts that were good fits made the adjustment period relatively quick; the more questionable fits were just distracting.