How to do a choice on aggregation with XML schema

Hello:
( Please redirect me to the correct list if this is not where I m supposed to 
ask this question )

Our application essentially sends xml 'commands' to another system.

These commands are essentially one xml element signifying which command it is ( 
3 types of commands ) and a corresponding data required by that command.
Because some different teams are involved, we do not have freedom to change the 
XML structure ( xml vocabulary )
XML appears in one of the following 3 forms

<COMMAND>REQUEST1</COMMAND>
<COMMAND_DATA>
<REQ1_DAT1> something </REQ1_DAT1>
<REQ1_DAT2> something </REQ1_DAT2>
...
</COMMAND_DATA>

or

<COMMAND>REQUEST2</COMMAND>
<COMMAND_DATA>
<REQ2_DAT1> something </REQ2_DAT1>
<REQ2_DAT2> something </REQ2_DAT2>
<REQ2_DAT3> something </REQ2_DAT3>
...
</COMMAND_DATA>

or

<COMMAND>REQUEST3</COMMAND>
<COMMAND_DATA>
<REQ3_DAT1> something </REQ3_DAT1>

...
</COMMAND_DATA>


We need to essentially build a schema that will restrict the xml to be in one 
of these 3 forms. Is it possible to do this at all.

I had trouble defining it as the definition for COMMAND_DATA is changing 
depending on what the COMMAND is.

I tried something like

<xs:simpleType name="req1Typ">
   <xs:restriction base="xs:string">
      <xs:pattern value="REQUEST1"/>
   </xs:restriction>
</xs:simpleType>

<xs:simpleType name="req2Typ">
   <xs:restriction base="xs:string">
      <xs:pattern value="REQUEST2"/>
   </xs:restriction>
</xs:simpleType>

<xs:simpleType name="req1Typ">
   <xs:restriction base="xs:string">
      <xs:pattern value="REQUEST2"/>
   </xs:restriction>
</xs:simpleType>

<xs:complexType name req1DatTyp>
   <xs:sequence>
      <xs:element name="REQ1_DAT1"/>
      <xs:element name="REQ1_DAT2"/>
      ...
   <xs:sequence>
</xs:complexType>

<xs:complexType name req2DatTyp>
   <xs:sequence>
      <xs:element name="REQ2_DAT1"/>
      <xs:element name="REQ2_DAT2"/>
      <xs:element name="REQ2_DAT3"/>
      ...
   <xs:sequence>
</xs:complexType>

<xs:complexType name req3DatTyp>
   <xs:sequence>
      <xs:element name="REQ3_DAT1"/>
      ...
   <xs:sequence>
</xs:complexType>


<xs:complexType name="choice_on_aggregation" >
   <xs:choice>
      <xs:sequence>
         <xs:element name="COMMAND" type="req1Typ">
         <xs:element name="COMMAND_DATA" type="req1DatTyp">
      </xs:sequence>
      <xs:sequence>
         <xs:element name="COMMAND" type="req2Typ">
         <xs:element name="COMMAND_DATA" type="req1DatTyp">
      </xs:sequence>
      <xs:sequence>
         <xs:element name="COMMAND" type="req3Typ">
         <xs:element name="COMMAND_DATA" type="req1DatTyp">
      </xs:sequence>
   </xs:choice>
</xs:complexType>

<element name"GLOBAL" type="choice_on_aggregation"/>


But though my xml editor ( xmlspy ) reported this to be a valid schema, when I used its 'generate sample XML' feature, it ended up generating xml that did not comply with the schema !!
When I used xerces to validate I got an error like

"http://www.bellsouth.com/wfaif":COMMAND and "http://www.bellsouth.com/wfaif":COMMAND (or elements from their substitution group) violate "Unique Particle Attribution". During validation against this schema, ambiguity would be created for those two particles"

Any help will be greatly appreciated
Thanks
--sony