You can redefine complex types.

The new definition must be a extension or restriction of the old definition.

This means a strange definition like:

<xs:complexType name="Person">
<xs:complexContent>
<xs:extension base="my:Person">
...
</xs:extension>
</xs:complexContent>
</xs:complexType>

Again, the effect is a type derivation and a shifting of all the uses to the derived type.