I've never tried this... it's just my theoretical response, in case there are no neater ways to do the task.

Option1 : Send yourself the 'old' thread but with the Mail Subject cleverly amended to read exactly like a reply to the 'new' thread. If you are on threaded view, I expect Gmail to exercise some stupid wisdom and append the 'old' conversation to the 'new'. Then you can send out the amalgamated email to the other recipients.

Option2 : Good old copy paste. This saves you substantial hair-loss. It's particularly handy when the recipients can take your word for truth.

Tony.