See https://addons.thunderbird.net/en-US/thunderbird/addon/edit-email-subject/

https://addons.thunderbird.net/en-us/thunderbird/addon/header-tools-improved/

Thanks, and I used the add-on. And it does allow me to change the subject line. But the email remained in the thread where it does not belong. How do I move an email out of a thread it does not belong in?

Try toggling mail.strict_threading to false in Settings/General, Config. editor. See if that works after Subjects are modified.

Otherwise, you might try modifying the References header with the second add-on above.

http://kb.mozillazine.org/Stop_threading_by_subject

Hi again. When I got to mail.strict_threading it was already set to FALSE. So I toggled it to TRUE and tried the name change and it worked!!! So thank you!!