What is legally permissible when adding a copyright notice to a derivative work?
In 2008 I stopped contributing to an open source project when it was pwned by a commercial interest. At that time a number of my original works existed in their source code repository, and still exist there pretty much as I left them 2 years ago, when I moved further development to a different repository. All of these works were released under the GPL, and carry my personal copyright notice, or that of a wider group of contributors who had worked on the project up to that date.
Since I left the project, the now-owners have redefined that contributors group, and have taken to retrospectively applying a different copyright to a number of these works. They have made some minor changes (mostly removing my personal branding, but also adding some scraps of new documentation) and added a dated copyright notice of their own that extends their claim back several years, despite the fact that (according to their own public records) no changes have been made until very recently.
Now, all the code and documentation is in their source code repository, so I would have no problem proving that I am the original copyright holder. However by making some very small changes, they have technically created a derivative work, and I don't have any problem with them adding a copyright notice to cover these new changes.
But what I find really objectionable is the dating of that new copyright notice back to a point well before the changes (5 years before, in at least one case).
This is obviously immoral, but the question is, is it actually illegal?
Update: it has come to light that in at least one case (not my code) the new copyright notice has been extended back to a date before the work was first published. I'm not sure whether to laugh or cry!