In an interview with the edgeGenndy Tartakovsky discussed handling the character of Aku without his voice actor:
“It was hard. For a while, I thought we should reimagine Aku in a different voice, maybe even use a different character. But then I realized I love Aku too much, so we auditioned some people, and Greg [Baldwin] he really did the best job. Honestly, no one can replace Mako, she was that special, that unique, especially the performance that she did for Aku. But Greg did very well.”
Tartakovsky ended up recasting the role, although replacing Aku with another antagonist entirely was considered. However, scrapping Aku’s character would completely change the series. “Samurai Jack” always had Aku at the center of Jack’s problems, and Jack defeating Aku when he returned to the past was the whole point of the show. Getting rid of Aku’s character would have significantly changed the trajectory of the show and Jack’s character compared to where they were when it first ended. Fortunately, Tartakovsky struck a fine balance of respect for Mako, who initially voiced Aku, recasting the role with an actor who brought a unique performance that harkens back to Mako’s original work on the character.