OK. "Express entry" is not a program, it is an application processing strategy. CIC calls it an "application management system."
FSW, CEC, FST, PNP... will all continue to exist (at least initially...). If you submit your express entry application, and receive the invitation to apply, you then have to apply through one of the economic class programs (you still have to meet the program requirements).
"Can somebody apply for FSW and Express Entry, and then withdraw the application from either one?"
I think what you mean is, can someone apply for FSW now, and then again later through the express entry process?
That's a good question... generally, you are allowed to apply for more than one program, but only have one application per program in process. I think you would have to withdraw one before you could submit the other - that is, if you still intend to apply through FSW when the express entry process starts. If you expect to qualify through a different program, such as CEC, then I think you would be able to do both.