It'll be possible, though, for a Jedi to do dark deeds from within the Jedi faction, and for a Sith to do good deeds from within the Sith faction. At least, that's what BioWare promised. From what I gather, you'll be able to choose how you act, but changing class and therefore faction, won't happen.
As everyone else, I think changing your class wouldn't be good. The story you get depends on what class you picked. Would be strange to get a whole other story because you changed class.