This Week's Getting Great With Google Tip

How to “Edit the Subject” line when Replying to or Forwarding a Message 
In addition to Reply and Forward, there is another option to edit the Subject line. Changing the subject line will start a new conversation rather than be attached to the existing conversation. 

To change the Subject line:
1. Click on “Reply” or “Forward” 
2. Click on the arrow to the left of the address you are sending to 
3. Select “Edit subject” a full compose window will open. From the full compose window, you can change the subject line.