Hello,
You do not need a new medical exam for a BOWP if you were exempt when applying for PR. The exemption criteria for a medical exam are the same for both PR and BOWP applications. However yo should be prepared to provide a copy of your previous medical exam results if requested by the immigration authorities. You should also note that your BOWP will only be valid until the expiry date of your medical exam.
I hope this answers your question helps you.
Best regards,
Stephen378Baker