Why does Tom Sawyer so readily agree to rescue Jim, when Huck has understood that Tom hates abolitionists? Is Tom changed by his efforts to save Jim?

Tom knows that Jim is already free....... he just doesn't let on. Tom is changed by his efforts because he's injured in the escape. He feels himself to be a hero, and he feels he has helped his best friend and done the right thing. Tom is changed by not only his own efforts, but by being a part of "the" effort.