In types.xsd:

   <xs:complexType name="Person">
     <xs:sequence>
        <xs:element name="name" type="xs:string"/>
     </xs:sequence>
   </xs:complexType>
   <xs:element name="person" type="ns:Person"/>

The redefine:

<xs:redefine schemaLocation="types.xsd">
    <xs:complexType name="Person">
       <xs:complexContent>
          <xs:extension base="ns:Person">
             <xs:sequence>
                <xs:element name="address" type="xs:string"/>
             </xs:sequence>
          </xs:extension>
       </xs:complexContent>
    </xs:complexType>
</xs:redefine>

This is now valid:

<person xmlns="http://www.example.org/test">
<name>Alex Milowski</name>
<address>San Francisco, CA</address>
</person>