title: Only change one property of ReplaySubject
body: <p>I have a ReplaySubject of type <code>UserDetails</code>. </p>
  
  <pre><code>userData: ReplaySubject&lt;UserDetails&gt;;
  </code></pre>
  
  <p>This class has the following properties with two being optional:</p>
  
  <pre><code>export class UserDetails {
    name: string;
    email: string;
    shipping?: Address[];
    billing?: Address[];
  }
  </code></pre>
  
  <p>To detect changes in other components via subscription, so far I've always used <code>this.user.next(value)</code> - but how do I e.g. solely change the property <code>shipping</code> of the object user?  </p>