The thought of making a recipe lighter or healthier always comes to mind. The lemon sauce is what makes the dish and I wouldn't suggest replacing heavy cream with any lighter alternative. Fresh lemon juice cuts down the richness, so no way the sauce will feel rich/heavy. Another reason is, heavy cream is high in fat content and doesn't split easily when heated or lemon juice is added. On the other hand half and half or low-fat cream have the chance of splitting.