Actually, I would say it's quite different than voice actors because outside of singing voices, it's usually the same actor voicing the role. Now if they were replaced in the sequel, that would be different. What we have here is an actor being replaced by another actor simply because they wanted a name actor to do the voice work for a pivotal scene, even though the original actor is going to voice the character both before and after this part.
You're making a big assumption here: that EA intended
for Mansell to only score the two scenes. Given the last-minute manner in which the other composers were announced, and the rampant inappropriate reuse of music from the last two games, I think it's just as likely Mansell was hired to score everything and dropped the ball entirely.