Getting a different actor to replicate the same cadence as the original (so that the speaking animation matches) can be difficult. I remember it was reportedly a challenge when they swapped out Peter Dinklage for Nolan North in "Destiny", and their character in the game didn't even have a mouth.
It can't be that hard. Delivering lines to match pre-filmed lip sync is something actors do routinely in ADR looping, since dialogue filmed on set or location often needs to be re-recorded for better quality.
Okay, maybe it's easier to replicate your own cadence, but I can think of several cases where an animated character's dialogue was re-recorded by a new actor. When Filmation recut and expanded their rarely-seen
Flash Gordon feature film into the first season of the TV series, most of the characters had their lines re-recorded by new actors, playing multiple roles each so that the series could have a smaller, less expensive cast. Similarly, when Hanna-Barbera extended their 5-part
Dark Water miniseries into the 13-episode
Pirates of Dark Water series, they couldn't get Roddy McDowall back, so his scenes in the first five episodes were revoiced by Frank Welker, who took over the character in the new episodes. There was also an episode of Marvel's
Avengers: Earth's Mightiest Heroes, I think it was, where Josh Keaton recorded a Spider-Man guest appearance, but
Spectacular Spider-Man was cancelled and replaced with
Ultimate Spider-Man before the episode aired, so they had USM's Drake Bell redo Keaton's lines (though lip sync is much less of an issue with Spidey).
And there are cases in live action where an actor's lines are looped over by a different actor because of performance issues or because the original actor isn't available for looping, like Gosheven in
Star Trek: TNG's "The Ensigns of Command." There have been cases where an actor has been redubbed by someone else because their accent was deemed too heavy, or not heavy enough. And there was Tim Burton's
Ed Wood, where Vincent D'Onofrio played Orson Welles but was dubbed over by Maurice LaMarche.