If you have a closed WP, you should file for an OWP with your inland application BEFORE the end of your current work permit, and you can keep on working (under implied status), until you receive the AIP ( approval in principle, first stage of inland application). Then they will process your OWP and send it to you. So if you're on a closed WP, you can keep on working throughout your waiting time, provided that you include an OWP in your inland application AND you file before your current WP expires. So if you're ready to file now, and your WP is valid until May, it's all good...
Good luck,
Sweden