HintonHQC,
When using TPH the value in the discriminator column is used when querying to determine the type of entity to create. Similarly, the type of entity is used to determine the discriminator value when sending updates to the database. Once an .NET object that represents the entity instance is created its type cannot be changed because .NET does not allow this. If EF allowed the discriminator to be changed then you would end up in a state where the type of object no longer matched the discriminator value. Furthermore, if you then queried for the object again EF would want to create an object of the new type, but then what should happen to the object that is already being tracked? Given these constraints on the way TPH mapping works it seems unlikely that we would allow the discriminator column to be modified through EF.
Thanks,
Arthur