Testing any future class level changes

So, I have a builder class like:

class Person{
private String age = "";
private String name = "";

public Person withAge(final String age)...
public Person withName(final String name)...

public Person build()...

// Now my question is due to this function
public Person copyWithSomeChange(){
Person person = new Person;
person.
    withAge(this.age+1).
    withName(this.name).
    build();
}
}

So, above class has a method copyWithSomeChange() which uses an existing Person object to create a new one.

Now I want to ensure if someone adds a new field to Person class, like SSN, then they doesn't forget to add that field to copyWithSomeChange() as well. I have created a unit test using reflection and did some hacks around that, but it doesn't seem quite right.

Am I taking a complete wrong approach here, I see this to be a good thing to do but how to do it the right way?