"Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer."
If you want to appropriate the code as GPL, you better do most of the work in new source files, or else change the file so significantly that (according to your lawyer) you're justified in replacing or altering the original copyright notice.
Anything that has only my name and my copyright notice on it is mine, even if it was modified. Whoever modified it acknowledged that it's mine by preserving the notices. It smells a lot like copyright assignment, and I suspect that very argument would easily succeed in court. I don't see how you can argue with a straight face that A. U. Thor is making an infringing use of your work, by taking an excerpt from a file which is "Copyright 2013 A. U. Thor".