Simply putting up a copyright, and a name of the current maintainer, _corporate employee_ who is responsible for maintaining the software, is not a large offense where I work. If you did not sign it at all, it could even be unsurprising that a newer developer would do so, to provide a contact point for users of the software, especially if hte copyright is a corporate copyright and not a personal one. They may even think they modified it enough to deserve a new copyright (which can be very easy to do), even if some of the best core components are essentially unchanged.
So there seems no need to start out heavy handed. Also, you're showing off in your interview that was done as a work for hire? Did you get permission from your former employer to display or share that work? Then you may be violating _their_ copyrights. So be safe: contact them, especially your old manager if you can find them, and ask for permission to show your old work, and see if you can cite them as a reference for doing that work.
If the new developer is actually plagiarizing your work and re-copyrighting it for themselves personally, your old employer is the one being hurt by this. Then you may need to show some traceable source control or software backups to enforce the claim. And you may be able to get cooperation from supervisors or HR at your old workplace. It could be awfully hard to sue for damages in a situation like this,, especially if you don't have good evidence. But someone who is plagiarizing your work will probably plagiarize other work, and a good manager will appreciate a heads up from the original author. This has happened to me and my colleagues before, and will again. It may be too late for you to follow good source code control practices, but those can be invaluable not only to locate who write the code, but who _broke_ the code later.
If you've got your evidence lined up, you might even be able to contact this developer directly and give them the opportunity to fix the situation. If they can provide a letter that says "this work was originally developed for Company A by _fill in your name_, and we're delighted with its performance.", I think you'd be in very good shape for the questions you w4ere asked.