Derivation by restriction wrt to type inheritance

Concerning derivation by restriction most samples are 'easy' samples
restricting complex types, e.g. by changing number of allowed occurences, or
simple types.

My question now is the following:
Is it allowed to change the type of an element "b" declared at complex type
"B", when deriving a new complex type "R" from "B" by restriction, and
setting the type of "b" to a subtype of its former type?

Or more concrete by the means of an example:
I define complex types Person and a complex type Staff (a subtype of
Person), furthermore a complex type for a list of persons, PersonList. I now
want to derive a new complex type StaffList from PersonList, restricting the
type of its only element "entry" from Person to Staff.

For me this is definitely a derivation by restriction, nevertheless IBMs
schema quality checker rejects it, and I suppose XML Spy has limitations
concerning derivation by restrictions (it accepts the schema below).

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified" attributeFormDefault="qualified"
targetNamespace="http://sample.org" xmlns:sam="http://sample.org">
   <xs:complexType name="Person">
      <xs:sequence>
         <xs:element name="name"/>
      </xs:sequence>
   </xs:complexType>
   <xs:complexType name="Staff">
      <xs:complexContent>
         <xs:extension base="sam:Person">
            <xs:sequence>
               <xs:element name="ssnr"/>
               <xs:element name="address"/>
            </xs:sequence>
         </xs:extension>
      </xs:complexContent>
   </xs:complexType>
   <xs:complexType name="PersonList">
      <xs:sequence>
         <xs:element name="entry" type="sam:Person"/>
      </xs:sequence>
   </xs:complexType>
   <xs:complexType name="StaffList">
      <xs:complexContent>
         <xs:restriction base="sam:PersonList">
            <xs:sequence>
               <xs:element name="entry" type="sam:Staff"/>
            </xs:sequence>
         </xs:restriction>
      </xs:complexContent>
   </xs:complexType>
</xs:schema>

Regards, Martin

I scanned the archives, but couldn't find messages to this topic. Sorry if
it had been asked before.