ultimate flexibility in schemas

Hi,
I am looking for an XML schema design which provides version
compatiblity between schemas. My schema has to evolve over time.
Newer versions of the schema would be created by the addition of new
elements (old elements are never removed)

Let's say, I have 2 schemas: version0.xsd and version1.xsd.
version1.xsd contains all the elements of version0.xsd and adds a few
new ones.
Corresponding xml documents are: version0.xml and version1.xml

I require the following combinations to get successfully validated:

version0.xml against version0.xsd
version0.xml against version1.xsd
version1.xml against version0.xsd
version1.xml against version1.xsd


Is there any way or am I asking for too much?

Thanks ahead.

Zombie