One Caveat: I'm not a lawyer so this is just my own experience.
But technically, your manuscript is already copyrighted. The current law comes into effect the moment of creation, you are not required to "submit" for a copyright. It still might be a good idea to register it at the Library of Congress though. It's not terribly expensive and it makes it a lot easier to "prove" that you developed the ideas/characters/etc, on your own.
If you then make drastic changes to the manuscript, you would have to resubmit to the archive.
Personally, it's easy to get caught up in the paranoia of protecting our ideas, but publication itself is a form of asserting your intellectual properties copyright.