Webservice change method response without notifying clients

Let's assume I have a webservice which returns a class on all methods, informing the client the status of the process, for example:

public class WsResult {

  string result; // either "error" or "ok"

}

Now we'd like to add a property to this class, without forcing all clients consuming our service to update their software. Is this possible?

For example:

public class WsResult {

  public string result; // either "error" or "ok"

  public Guid? someIdentifier;

}

I'm looking for answers on both WCF and ASMX.