And yet, I'd forgotten it. I'd argue that it's actually pretty easy to switch a character like that when you have the person right there to show you how and have been working with them for 30 episodes.
Adam Project comes to mind. A child actor was able to perfectly imitate Ryan Reynolds as a child. Not exactly a high bar for a kid who grew up with Deadpool and ... well Ryan only has the one character, but a preteen novice actor pulled it off.
Farscape also has the problem of I didn't watch it back when the effects were more cutting edge. They felt really cheesy and it hurt the believability of the show imo.
Like I said ymmv.