I'm not sure you can say that 2 characters change--seems to me that Huck is the only dynamic character in this story and that the rest of them stay static or unchanged.
Huck goes from a simple child that does not question the morality and/or ethics behind the practice of slavery to having an adult realization that slavery is wrong. Along this journey, Huck realizes that just because everyone is doing it doesn't mean it's the right thing to do. He begins to question the practice of slavery because he begins to see Jim as a human being, as a father figure, and as a friend. Huck feels so strongly about ensuring Jim's freedom that he even is willing to go to hell for being an "abolitionist"--the worst thing a southern man can be.
I guess you can say that Jim changes, but it's not really him that changes, it's more Huck's PERCEPTION of Jim that changes (and the reader's perception too). In the beginning Jim is portrayed as this comical, ignorant slavehand, but as we get to know him we learn about his family and about how much he cares for Huck. The reader and Huck get to see Jim's personal side. But as I said, Jim was this way all along--always caring, helpful and kind--so it's how Jim is viewed that changes, but not Jim himself.