[plt-scheme] Upwards and Downwards Compatible Object Serialization

I'm using serializable objects defined by the new 
define-serializable-class. Works great! But ultimately, my classes will 
have to implement externalizable<%>  in order to support either SXML or 
XML as a generic data format.

Now I can't figure out how to make such data upwards and downwards 
compatible. By this I mean the following. My application v1.0 starts 
with using class A and externalizes it. Then application v1.1 defines a 
subclass B of A, which adds a lot of fancy additional fields to A, but 
inherits everything else from A. I want application v1.0 be able to 
read any externalized instance of B as if it was data of class A, 
simply skipping any additional v1.1 stuff. Likewise, v1.1 has to be 
able to read externalized objects of class A as instances of class B, 
where the additional fields are given their default value. And so on 
for all other versions.

Any solution I've come up with so far seems to be clumsy and error 
prone to me. Perhaps "I can't see the wood because of all the trees" as 
a German proverb says. I'm looking for advice and ideas how to 
implement this functionality. Any suggestions?

Best regards,

Erich