After more than year since penguin, I have finally decided that those sites are not repairable and they will probably be really low in google rankings forever. I'm prepared to lose all the links I have built to them, I want to start again, from zero. Is just changing domains enough, or will google still consider those sites which still have the same content and are on the same hosting, as the same sites that had just different domains? Or will it be like fresh start? thanks for answers.