I'd try this outside of Thunderbird. Close it, locate the appropriate folders in the profile, under the old account, and try copying them to the corresponding location under the new account. If it's POP I'd be highly confident of this working. With IMAP, I'd hope that the server would see the new files and immediately set about synchronizing them to the server. I'd also hope that you have the full messages locally cached, so what you upload to the server represents the entirety of the old messages. When I was recently experimenting with exporting messages in bulk, I got warnings about incomplete messages. :-( A halfway house would be to copy the old folders to the Local Folders in Thunderbird, so the server need not become involved. But that won't help if you don't have full local copies. I think moving your folders/messages (in Thunderbird) to Local Folders (from the old account) is the most sure-fire way of getting them all copied locally and complete. But tiresome and slow.