, No, you're wrong.
I have a thread with the subject "Happy Birthday".
I always open it, hit "Forward" remove the FWD prefix, delete all past text, write a new email address and hit send.
The new sent mail is added to the current thread under the subject "Happy Birthday".
This way all my conversations with people about their birthdays is in one place.
It's really a great way to sort emails.
Can't be done with the new compose.