No you can't. Well, due to fair use you probably could without losing a legal battle over it, but for more substantial improvements you couldn't do that. Presumably you thought about this when choosing the BSD license (or when choosing to contribute to a BSD licensed project).
I believe that any improvements which are such that a source file still has only my name and copyright notice on it fall under the original license.
The original license forbids changing the license:
"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".
There's nothing stopping a contributor from keeping your copyright notice, and adding his own license that only covers his own modifications. The contributor does not technically need to change the license of _your_ work, but he is free to chose whatever license he likes for his own changes to your code.
