Nimoy's role was most likely the one that needed a rewrite the most, but the writer's strike precluded that.
Why does Kirk tell Old Spock that coming back in time and changing history is cheating, to which Old Spock says it was a trick he learned from an old friend, when according to the film, Spock came back in time by accident? Yeah...needs, or was already in the middle of, a rewrite.
I didn´t get this at first, but I think he is referring to the Kobayashi Maru test where Kirk cheated.