We have an existing ServiceContract
[ServiceContract(Namespace = \"http://somesite.com/ConversationService\")]
public interface IConversationService
{
I think your existing clients will continue to work. After all this is very similar to SOAP and web services in that the client will connect to the given URL and request a specific service. If you take methods away you will risk breakage (only if the method is used I believe) but adding should be pain free.
I've only dabbled in WCF but have used the ASP.NET web services in this way to great success.