Did you update account settings for both incoming and outgoing servers?

Hi Stans. Yes, as far as I can tell. The SMTP is in the "main" account settings page (and also under the "Outgoing Server SMTP" page - both reflect the change) , and the IMAP is under the "Server Settings" for that account page. Is there perhaps something that I have missed?

Open your profile folder and Quit Thunderbird. Inside the folder, you'll find a file named prefs.js (the .js extension may not be visible depending on your system settings). It's a plain text file which you can open using a text editor like Notepad. Open it with Notepad and use the Find function to locate any remnants of the old account settings, for example, the IMAP server name. Replace those entries with the new settings OR delete the line containing those old settings then launch Thunderbird.

Thanks so much. I've found it, and think it's up and running. I'm still baffled as to where it saved the settings I changed in the GUI, because it was persistent between instances. Anyhow, up and running, thanks!