Thunderbird Plugins – Modify IMAP Emails

Sometimes I receive emails with subjects like "As discussed" or "(empty)". At that point I know what it is about but for later reference I'd like to change the subject into something slightly more useful. The emails are accessed via IMAP, thunderbird can basically modify them (as the detach attachments feature shows), so is there a way to modify the subject line as well?

edit saving a modified draft is not what I want, since that changes the sender to myself and the date to the modification date