Drain pineapple juice into a saucepan. Add margarine and lemon juice; cook over medium heat until margarine is melted. In a small bowl, combine cornstarch and water until smooth; stir into juice mixture. Bring to a boil; boil and stir for 2 minutes. Reduce heat; add sweetener. Cool to room temperature, about 30 minutes. In a large bowl, combine pineapple, apples, grapes and poppy seeds. Add the dressing; toss to coat Cover and chill for at least 1 hour. Just before serving, gently toss in pecans.