That's not an alteration of the original, the original (but isn't The Empire Strikes Back a part of the original anyway?) was simply Kenobi's retelling of what happened, not a narratively "objective" instance. Vader telling the story in a different way does not create any fundamental contradiction with the previous narrative.

Anyway, the original SW trilogy is much too homogenously constructed to warrant this sort of criticism in general. It's like saying Sophocles "retconned" Oedipus' story by revealing he had killed his father. A more problematic point would be e.g. the introduction of midichlorians in the prequels, which didn't unambiguously contradict the original trilogy but it sharply differed in spirit from it and had undesirable implications (genetic superiority of the Jedis).