How does one modify the discriminator value in the database?

I have a class Client and a subclass BillingContact. They are both persisted in a single Users table with a discriminator column whose value is either 'client' or 'billingcontact'. The distinction between the two is that a BillingContact has a mapped BillingAccount property while a Client does not. I need to be able to turn a Client into a BillingContact or vice versa.

Any help is much appreciated.

Thanks,
Mike

Hibernate version: 2.1