Can I modify a registered work?
Once a work is registered neither the title, author or content of it can be modified as the registration proof is based in the fact that "that" is the precise works that was registered at a specific time and UTC time.
However, any modification is susceptible of new registration, though it is recommendable not to create confusion to the possible users by registering too many times the same work with no substantial changes. Whenever the author of a work makes any kind of change on it and registers it again, it becomes a "derived work", by the 11th article of the Law of Intellectual Property (Spain).
Regards the title of a modified work and registered again, the Law does not prohibit to keep the same title as the first version. First because, being the same author, the only damage, as previously said, might the possible confusion effect it can bring to users. It could be though, that two works from different authors have the same title. In that case, the Intellectual Property Law (Spain) in it's 10.2 article, says that a title is only protected whenever it's original. Is quite commons and perfectly legal that two authors have each one a song with the same title "The Dawn". On the other hand a title containing, for instance, the name of one of the main characters of a the book like "Alice in Wonderland" would be considered as original and thus protected by copyright laws.
What you can change:
* The license of the work.
* Whether the registration is public or not (if others can see who the author is and he's rights).
* Whether download of the work is available or not (if other, when consulting the rights, will be able to download the work)