Same Phone Number - Upgrade & Downgrade
Phone numbers deleted from a WhatsApp Business Account (WABA) cannot be used again for another account Source: https://developers.facebook.com/docs/whatsapp/overview/phone-number#delete-a-phone-number-from-your-whatsapp-business-account

Same Phone Number

Start from WhatsApp Messenger/ WhatsApp Business/ Not Linked to any WhatsApp Account:
✅ -> CAN Upgrade to WhatsApp Business API
✅ -> CAN Downgrade to WhatsApp Messenger/ WhatsApp Business
❌ -> CANNOT Re-Upgrade to WhatsApp Business API

Downgrading

If you wish to downgrade to the standard WhatsApp App or the WhatsApp Business App - you must log into Facebook Business Manager and follow the deletion procedure.
If the WABA is not shared with you (you cannot see it in your WhatsApp Manager) - please create a support ticket from 360dialog and request for number deletion for you.