Do you have any advice how to best move the old emails to the new IMAP account? My current provider started getting flaky when they were bought out by a larger

Do you have any advice how to best move the old emails to the new IMAP account? My current provider started getting flaky when they were bought out by a larger one, but I've been putting it off for years trying to move. Maybe offlineimap to a local box, probably best to have your own backup anyway.