How can I change the Subject of an forwarded email

I have the new email service which has the conversation thread which is very nice. Something that I was able to do with the old email was change the Subject of an email bewfore replying or forwarding. There are times when I wish to use all the email addresses listed in the cc to address another subject. Opening an older email with these email adresses and changing the Subject makes this easy. I can not do this with the new full featured email. Also copying and pasting addresses does not seem to work either. How can I change the Subject in an email before forwarding or replying?