When changing from IMAP to POP, do not delete the IMAP account first. Add the POP account first, then you can move your emails from the POP account to the IMAP account. Then after all email is moved, delete the IMAP account. See IMAP>POP page to change your email account from IMAP to POP.
An alternative Microsoft Outlook IMAP to POP help page: https://www.msoutlook.info/question/705
IMPORTANT: Make sure to use POP