when does one change date of copyright?

Question

What percentage of text needs to be changed?

Answer 1

Here's what the US Copyright Office has to say:

"How much do I have to change in my own work to make a new claim of copyright?

You may make a new claim in your work if the changes are substantial and creative, something more than just editorial changes or minor changes. This would qualify as a new derivative work. For instance, simply making spelling corrections throughout a work does not warrant a new registration, but adding an additional chapter would. See Circular 14, Copyright Registration for Derivative Works, for further information."

Hope that helps!