JAE,
I work for an engineering firm. In our proposals, we very clearly itemize the deliverables which will be issued: a report, drawings, specifications, etc.
Consider a case where an Owner has an existing device or structure which my company designed. The Owner wishes to enhance the device, or add something to the structure (maybe add a monorail/hoist in a particular location). The Owner could contact my company to engineer this work, but is perfectly within his rights to go to another company for the design. In so doing, the Owner would have to provide the plans and specifications of the device or structure (prepared by my company) to the "new" engineering company.
If the Owner does not have ownership of the design documents, then I guess we, and our clients, have been doing it wrong for a long time.
I agree with others here that the issue in your original post isn't so much one of ethics, as it is of liability. The new engineers will have to assure themselves that the work done to date was correct.
In the jurisdictions in which I am licensed, I am ethically obligated to inform other engineers if I am reviewing their work provided they are still involved with the project. It tends to get a little hazier if they are no longer involved, but the recommended practice is to inform them even after they are no longer on the project.
Cheers,
CanuckMiner