I think you are confused with how copyright works.
Copyright is granted to an author automatically when an author publishes a work. The author can perfect those rights and gain more rights by registering the copyright.
http://www.copyright.gov/eco/ you can follow the instructions here to register your copyright.
Copyright only protects reproductions of the work. It does not protect the underlying ideas in the work. So while someone could not photocopy and sell your TV treatment, copyright will not allow you to prevent others from making a show with exactly the same plot and ideas, provided the dialogue is not a word for word reproduction.
You should consult with an entertainment attorney to see how best to protect your TV treatments.
You could file a new copyright, its not expensive. But just because you copyright a treatement does not mean that the idea for the treatment is protected. You should consult with an entertainment attorney.
Ivan J. Parron, Esq.
My colleagues offered good insights. If the changes are not very drastic or substantive this is likely not necessary. If, however, your final version is quite distinct from the material you previously registered, I would simply send in a new filing. Remember, the date of filing has important legal significance, and if the material has changed you cannot be given the prior date of registration for different material so a new form will need to be submitted.
You may want to discuss your plans in private with a lawyer in more detail. Most of us here, including myself, offer a free phone consult.
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As my colleagues explained, it depends on your need to protect it legally to enforce the copyright. As you already registered your first draft, you already know the cost to file. Online filing fee id $35 for a basic claim.
This is not a legal advice or solicitation, and does not create an attorney-client relationship. Consult with an attorney. I work for Cardinal Risk Mangement and Cardinal Intellectual Property, IP service companies, but not law firms. I also am the president of Vepachedu Educational Foundation Inc., which is a non profit educational foundation. I also write cultural and scientific compliations for the foundation. I also teach at Northwestern university as a guest lecturer. I also provide some pro-bono guidance on immigration and other issues through Indian American Bar Association. I also have a contract with Cardinal Law Group, a law firm, for IP projects. All this information is on my profile at Avvo and also at Linkedin. Any views/opinions expressed in any context are my personal views in individual capacity only, and do not represent the views and opinions of any firm, client, or anyone else, and is not sponsored or endorsed by them in any way.
As Attorney Naatoli notes, filing a new registration application is necessary. There is no mechanism to file an "addition" to an existing work. When you apply to register the copyright in the new work there will be an entry on the form to identify any "pre-existing" work on which the new work was based. That is where you note that the second draft is an amended version of the first. To keep things simple, you should title your second draft as "Title of Original, Version 2" [or something similar to designate it as an amended version of the original. You should speak with your own copyright attorney beforehand to get more clarity on copyright registration matters. Good luck.
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.