All the new material in a rewrite can be covered with a revised filing. If you elect to not copyright your first draft, you should get copyright coverage for your finished screenplay prior to beginning your marketing efforts. Registering a copyright for your script is proof of ownership and puts a copy of your screenplay in the Library of Congress. Information on copyright and the appropriate forms are available at the Office of Copyrights. The Wrtier's Guild Registration Service allows you to register material.
Some people advise that a first draft and a finished screenplay be registered.
There are possible advantages in registering ideas with the Guild. Be advised that mailing a copy to yourself provides little protection. Use the WGA Registration Service for material which cannot be granted a copyright and copyright the material which can. You can gain an overview of copyright law by reading Circular 1 here. This article is one of many here at Screenwriting Help.