I think being POP means there aren't any issues. It's all stored locally, so no complicated interactions with servers.

With IMAP, each folder has a live connection back to its server. Moving folders between accounts looks simple but involves copying messages between Thunderbird and the various (two or more) servers involved. I have heard it reported that a folder moved from one account to another retained its allegiance to its original account. For this reason I usually suggest moving the contents of the folders, not the folders themselves. This takes longer but it doesn't (often) go wrong.

But if you're not killing off the old account, what's the harm in leaving the messages in it?

I don't know if it works with folders, but try a ctrl+drag-and-drop to set up a move rather than a copy.

I actually use an add-on that emits a pop-up and nags me if I try to drag a folder. ;-)

Hi Zenos - I appreciate your reply. The reason for moving them to the new account is to enable me to look in one folder for an email, rather than two, particularly during the changeover period.

I will take your advice, and just setup duplicate folders in the new account, then move the messages (and finally delete the old folders). A longer process, but if it is safer, that's how I'll do it.

Thanks.