<div ng-controller="MyController" >
    <button ng-click="changeAll()">Change</button>
    <ul>
        <li class="item" ng-repeat="i in myList">{{i}}</li>
    </ul>
</div>

var MyApp = angular.module('MyApp', ['ngAnimate'])
MyApp.controller('MyController', ['$scope',function($scope){
    $scope.list_a = ["Item A","Item B","Item C"];
    $scope.list_b = ["Item D","Item E","Item F", "Item G"];
    $scope.myList = $scope.list_a;

    $scope.changeAll = function(){
        $scope.myList = $scope.myList.length == 3 ? $scope.list_b : $scope.list_a;
    }
}]);

.item.ng-enter,
.item.ng-leave{
    transition:all 1s ease;
}

.item.ng-enter,
.item.ng-leave.ng-leave-active{
    opacity:0;
}

.item.ng-enter.ng-enter-active,
.item.ng-leave{
    opacity:1;
}