Lets say I have a project which has evolved over the past year. When I began the project, I looked around for open source projects which provided as much of the base system as I could find, and finally settled on an Apache 2.0 Licensed project.
Over the past year, the project has evolved a lot, leaving very little resemblance to the original project. Basically the only similarities left are the project's source directory structure, and ideas left in-tact here and there (very few complete class files are original, most are either totally re-written or mostly due to this project's requirements).
At what point do I stop attributing to the original project? At what point have I diverged so severely as to be safe in saying I own copyright over this entire work as well as licensed under my terms?