I take it you've tried the obvious and simply changed the discriminator column, updated the entity and done a "em.refresh" on the entity?
I can see, potentially, how that may not work, but I just want to make sure you tried it. But in theory, you'd like to think that when you do the refresh, TopLink will reload the instance, see the new discriminator, and then automagically create the new class.
Worth a shot at least.
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