By changing, I assume this is something like switching from donna@abc.com to donna@xyz.com. You can save the messages and folders by - exiting thunderbird - in windows file explorer, within the Mail folder and account folder, copying the message folders (e.g., INBOX, INBOX.msf) to the Mail\Local Folders folder. - restart thunderbird and confirm that the folders are readable. - you may now delete the old account. your addressbook remains intact. Nothing is lost.