ReallyUnsafeSetType.cs

using System;
using System.Runtime.CompilerServices;
using System.Runtime.Serialization;

class Program
{
    static void Main()
    {
        var x = new Person();

        Console.WriteLine(x.GetType());
        ReallyUnsafeSetType(x, typeof(Dog));
        Console.WriteLine(x.GetType());
    }

    unsafe static void ReallyUnsafeSetType(object it, Type t)
    {
        // just need it for its typecode, so don't bother initing
        var target = FormatterServices.GetUninitializedObject(t);

        // treating as string lets us pin, it's a magic type
        // otherwise you can only pin blittable types
        fixed (char* cIt = Unsafe.As<string>(it))
        fixed (char* cTarget = Unsafe.As<string>(target))
        {
            // copy across typecode (lives 3 ints before first character)
            *(((int*)cIt) - 3) = *(((int*)cTarget) - 3);
        }
    }
}

class Person { }
class Dog {  }