Changing the type of an object inside a method?

First off, this is not my code. I'm curious what everyone thinks about changing the type of an object inside of a method then modifying the properties of the cast type. I wanted to use composition to build up a patient, but this is the recommend path. I'm probably wrong which is why I wanted 3rd party feedback. Or maybe we just have different coding styles...

// Person/Patient Builder
public static void BuildUpPersonOrPatient(Person person, Dictionary row)
        {
    person.FirstName = row["FirstName"];
    person.LastName = row["LastName"];

    Patient patient = person as Patient;

    if (patient != null)
    {
        patient.PatientID = row["PatientID"];
    }
}

for reference

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class Patient : Person
{
    public string PatientID { get; set; }
}

//
//

It stead of having a static method the basically does what polymorphism could do for you use polymorphism.

public class Person
{
    public string FirstName { get; private set; }
    public string LastName { get; private set; }

    public virtual void InitializeFromDictionary(Dictionary row)
    {
        FirstName = row["FirstName"];
        LastName = row["LastName"];
    }
}

public class Patient : Person
{
    public string PatientID { get; private set; }
    public override void InitializeFromDictionary(Dictionary row)
    {
       base.InitializeFromDictionary(row)
       PatientID = row["PatientID"];
    }
}

and then where ever you call

BuildUpPersonOrPatient(p,row);

do this instead

p.InitialieFromDictionary(row);

I've added an access modifier to the setters because unless you have a reason to be able to change those fields from outside the object you should encapsulate the possibility of chaning the state of an object. (It's surprising how seldom you have a modeling reason for that and not just a "for conviniece" reason)

StackExchange Code Review Q#56687, answer score: 7