WebServices, PersistentClasses and deployment of changes

Hi,

This topic is a little bit off topic (because it concerns ORMs in general, not only OpenAccess).

The context of the problem is that we have:
- a WebService which serves ChangeSet objects
- a PersistentClasses library
PersistentClasses is used by WebService. It must be used by every WebService's client who wants to use ChangeSet objects served by the WebService.
And one day we add a column to one of the tables. We update our PersistentClasses library and the WebService. And we send the new PersistentClasses to all WebService's clients. But if we miss any client - this client stops working because it has out-of-date PersistentClasses library. Ugh. The more the idea of webservices (one of the ideas) is that we needn't know (and don't know) all their clients.
The root of the problem is that we use this WebService in an improper way; improper because the PersistentClases library is out of the contract. But as I understand there is no better way.

So - what is the solution of this problem? Or maybe it is not a problem but my imagination only :-) ?

Thanks for all your comments.

Regards
Tomasz