When removing an account, there is a prompt on whether to keep or delete message files. When in doubt, however, it's never wrong to create a backup prior major actions.

Thanks David, this helps.

It appears old messages from POP were automatically migrated to the IMAP Local Folder.

When converting from POP to IMAP you're setting up a new IMAP account in Thunderbird. The messages you see in the new account is what's already (or still) on the IMAP server. Nothing is being converted automatically. That would be the case if the setting for your POP account was set to 'Leave messages on the server'.

how can I confirm that before I remove the old POP account?

What you need to confirm is whether you downloaded messages via the POP account and deliberately deleted them on the server. Those wouldn't show up in the IMAP account.

The conversion instructions said I needed to manually move the old messages, but they are already in the Local Folder.

That would apply to messages which are local only, and not on the server anymore.