I don't understand the comment about forking. Sure, you can't take my files and change the license on them. But you can take any and all of my files as is, incorporate them into your own GPL project. That's effective the same thing as now the project itself must be distributed under the terms of the GPL. So long as you don't violate the terms of the BSD license, you are free to do what you want. Going the other way is not possible. (Of course when I say BSD license I mean modified-BSD, not the original 4-term license. We don't use that of course.)