In an interview with Vanity Fair, Lightyear's director Angus MacLane said that the change in actors was made because of the differences between this movie and the Toy Story franchise. "Tim’s version of Buzz is a little goofier and is a little dumber, and so he is the comic relief," Angus said.

He continued: "In this film, Buzz is the action hero. He’s serious and ambitious and funny, but not in a goofy way that would undercut the drama." Angus went on to say that Chris has the "gravitas and that movie-star quality" that superhero Buzz needed to make Lightyear stand out from Tim's version of the same character.

In addition, Chris Evans himself posted on Twitter about taking on the role of Buzz in Dec. 2020. "And just to be clear, this isn’t Buzz Lightyear the toy," the tweet said. "This is the origin story of the human Buzz Lightyear that the toy is based on."

Because of the wording in this tweet, it's clear that Tim isn't being replaced. It's very possible that if Buzz (the toy) makes an appearance in an upcoming Toy Story project, Tim would come back to voice our favorite space ranger.