The good news is that IMAP auto populates. That is, when you reinstall an IMAP account, all messages immediately appear. There is no need to copy them for the purpose of moving to new PC. This is because the messages reside on the server and only headers on the PC.

Not sure if I made it clear enough in the original question, sorry - this is actually about moving emails from one server to another. I need to put the messages somewhere else while opening a new server which will then be keyed to the same account details, but will not have the messages.

I think I've figured it out though, I'm gonna use local folders on Thunderbird to hold the messages from the old server, and then on connecting to the new server I'll copy them over to that.

Ok, now I understand. I concur to your strategy with my comments: - first, check account settings>synchronization&storage to verify that all folders can be downloaded - now, click file>offline>download to ensure that all IMAP messages are fully downloaded to the PC - I would then exit thunderbird and copy the message folders to external drive via windows file explorer and then move them to the Local Folders area in profile. I can give details on that, if needed. - now, after creating the new IMAP server account, you can upload the saved messages. Since IMAP was not designed to do that dance, it can be problematic, so I suggest creating a manual filter to copy messages from local folders to the IMAP account, and then to spot check to ensure that the transfer was successful.