He evolved the character.
I don't know how Nimoy's increasingly worse portrayal of the Spock character qualifies as "evolution." After STAR TREK II, Nimoy stopped putting any real effort into his performances.
I couldn't disagree more. Nimoy's interpretation of the character changed by both the experiences he had during the movies and the natural changes brought on by age. Old Spock is at peace and in control, having united both of his halves, valuing both logic and emotion but a slave to neither. Finally whole.
Also, like most elderly people, he just doesn't give a shit anymore about what other people think of him.