i will assume this is an IMAP account. If not,let me know, as steps would be different. You can export your IMAP messages with these steps: # verify that the account can be downloaded at 'synchronization&storage' pane and click 'advanced' tab # then, check that you have 'sync regardless of age' at same pane # next, click file>offline>download to ensure you get all content of all folders # next, click help>troubleshootinginformation # scroll to 'profile folder'' and click 'open folder' # EXIt thunderbird. you are now in the profile # click the imapmail folder and then the IMAP account folder # Look for the message folders, identified by the combination names such as inbox, inbox.msf, sent, sent.msf, etc. IGNORE the msf entries, but COPY all of the others elsewhere # start thunderbird # install the 'importexport NG addon # highlight Local Folders # righclick and select the importexport option to import mbox # locate and select one of the exported message folders, such as 'inbox' # repeat the prior two steps for each exported message folder # you should now see all account folders in Local Folders with their content. Check that content is there. # You now have saved all account messages locally and, once you confirm to yourself that you have everything, you can remove that account. # after setting up a new account, you are ready to COPY the saved messages to the new account. After new IMAP account is set up, do the following: # Create the desired folders in the new account # For each folder, select messages in the Local Folders folder, and select the 'COPY to' option. IMAP was not designed for high-volume copying, so you may need to do that in batches. Or create a manual filter to do the COPY. Repeat for each folder. CHECK to ensure all messages are successfully copied. # once satisfied, you may delete the Local Folders folders.