Whoever starts working on adapting an existing open source project (or creates his own) eventually will get to the question about how to deal with existing copyright notes and whether and how to add ones own to existing (or new) files.
The first question is: Are copyright notes legally required?
The short answer to this question is: No. Legally the copyright notes carry no weight. They can actually be completely omitted from source code and ones own work, without impacting the fact that the work is still under the author’s copyright. However, copyright notices can help and are easy pointers for everyone to get the copyright information.
I found a really nice article while googling for the question from Ben Balter  who gets into some more detail on the topic.
I for myself have therefore decided to always add my own copyright markers to source code files . Since I normally just take over the existing license of the original source code, I simply add my copyright just behind the existing copyright notes. (Note: this is due to my changes normally being just minor compared to the existing work and hence I want to support the idea of the original author by ensuring my modifications are covered by the same freedom he offered his own work for).
For source code not maintained in a publicly accessible version control system, I also add a note about the changes I did, so everybody can determine which part of the source code (or which modifications) are covered by my copyright in contrast to everything else, which is covered by the original author’s copyright.
Bear in mind that different licenses might however have different requirements on how to relicense your modified source and how to deal with existing copyrights.