Well, without getting into too many details, he'd be replaced by a general that wasn't nearly as brilliant as he was and the ACW would end a little earlier. Although if his death butterflied away Stonewall Jackson's (which took place two months later in May of 1963), that could make for a more interesting timeline, since Jackson would be Lee's logical successor. I don't know if it would be sufficient to change the outcome of the war though. If I did a little research and knew a little more about military strategy then I could be more specific.