I just finished readig an article by Thomas J. Stueber in The Internet & Copyright Lawyer: Due Diligence in Drafting: Copyrights in Legal Documents.
He takes that position that just about everything created by lawyers in the ordinary course of their practice are, for copyright purposes, "literary works." Artistic merit is not a determination of whether something is protected by copyright. It nearly need to have "some creative spark, no matter how crude, humble, or obvious." Feist Publications, Inc. v. Rural Telephone Services Co, 499 U.S. 340 (1991).
A lawyer who rewrites a document creates a derivate work. Creating a derivate work without permission is copyright infringement.
Employees creating copyrightable work during the course of their employment are creating works for hire, which vest the copyright in the employer. So the law firm owns the copyright for the lawyer’s original work. Or for in-house counsel, the company holds the copyright. In most cases, the lawyer is an independent contractor for the client, so the client does not hold the copyright.
If an attorney switches law firms, the copyright stay with old law firm by default. In taking forms and precedents with you, even those you drafted, you could be infringing on the copyright.
If the substantial portions of the document are from pre-existing sources that you did not draft yourself, the document may not be sufficiently original to be copyrightable.
I think most transactional documents have been passed around and collaborated on across firms that it would be difficult to prove that sufficiently original to be copyrightable.
Some legal briefs may be original enough. Milberg Weiss has tried to enforce a copyright on its pleadings. Milberg Weiss Tries to Nail Class Action Imitators