[sebiol]

I understood the advise to only apply to emails you forward not to emails you reply to.

With replying I would be careful about changing the subject line too. With a forward I send it to new people, as if it were a new mail. In this context it makes sense to me you might also give it a more appropriate subject line.

For example you get a newsletter. One news item is relevant for a friend of yours. You forward the newsletter and with the subject line you direct his attention to the item that is most relevant for him.