Mutating Expression Tree

Program.cs

public class Program
{
    private static void Main()
    {
        var operandOne = Expression.Parameter(typeof (int));
        var operandTwo = Expression.Parameter(typeof(int));

        var expression =
            Expression.Lambda(
                Expression.Add(
                    operandOne,
                    operandTwo
                ),
                operandOne,
                operandTwo
            );


        var visitor = new AddToSubtractExpressionVisitor();
        expression = (LambdaExpression)visitor.Visit(expression);

        var func = expression.Compile() as Func<int, int, int>;
        Console.WriteLine(func(10, 10));
    }
}

public class AddToSubtractExpressionVisitor : ExpressionVisitor
{
    protected override Expression VisitBinary(BinaryExpression node)
    {
        return node.NodeType == ExpressionType.Add
            ? Expression.Subtract(
                this.Visit(node.Left), this.Visit(node.Right))
            : node;
    }
}