It used to be impossible with TB to switch an account from IMAP to POP3, but I don't know whether that restriction still goes.
However, I'd suggest that you create a new account with POP3 and delete your IMAP account (as configured in TB) afterwards.
Keep in mind that POP3 doesn't support multiple folders on the server side, so if you use folders in your IMAP account you should move those messages to your POP3 account manually.
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I'm just a user of The Bat! I don't work for Ritlabs.