title: Kafka schema registry not compatible in the same topic
body: <p>I'm using Kafka schema registry for producing/consuming Kafka messages, for example I have two fields they are both string type, the pseudo schema as following：</p>
  
  <pre><code>{"name": "test1", "type": "string"}
  {"name": "test2", "type": "string"}
  </code></pre>
  
  <p>but after sending and consuming a while, I need modify schema to change the second filed to long type, then it threw the following exception:</p>
  
  <pre><code>Schema being registered is incompatible with an earlier schema; error code: 409
  </code></pre>
  
  <p>I'm confused, if schema registry can not evolve the schema upgrade/change, then why should I use Schema registry, or say why I use Avro?</p>