How do the two characters change/improve due to these strong and aggressive personalities?

McMurphy and Ratched

With the help of McMurphy, Chief Bromden begins to speak once more and reasserts himself against Nurse Ratched and her workers. Ultimately, he breaks free of the Nurse and the hospital, killing the lobotimized McMurphy in order to prevent him from suffering further indignity, and finding his way back into society away from the repressive manipulation and tyrannical authority of the institution.

Charles Cheswick, one of the patients on the ward, is one of the first patients to support McMurphy. McMurphy helps him discover sense of personal identity. He challenges the status quo. Unfortunately he is taken to the Disturbed Ward, presumably for shock treatment, when he starts to protest the ward policies.