How to convert one type object to another - change type ( class )

using DefineAnInterfaceIPerson;
using System;

namespace PersonInfo
{
   public class StartUp
    {
        static void Main(string[] args)
        {
            string name = Console.ReadLine();
            int age = int.Parse(Console.ReadLine());

            IPerson person = new Citizen(name, age); // IPerson don't have method "SayHi()"
            Console.WriteLine(person.Name);
            Console.WriteLine(person.Age);

            Type type = person.GetType(); // how to change from one type to another
            Citizen citizen = (Citizen)Convert.ChangeType(person, type);
            citizen.SayHi();
        }
    }
}