For a novel about memory, the plasticity of the novel’s narrative was one of its most realistic elements. Proust was always refining his fictional sentences in light of new knowledge, altering his past words to reflect his present circumstances. On the last night of his life, as he lay prostrate in bed, weakened by his diet of ice cream, beer, and barbiturates, he summoned Celeste, his beloved maid, to take a little dictation. He wanted to change a section in his novel that described the slow death of a character, since he now knew a little bit more about what dying is like.
That is from the new and quite interesting Proust Was A Neuroscientist, by Jonah Lehrer.