To register WhatsApp Business API, you can use your current WhatsApp or WhatsApp Business number, but you have to delete the previous account that has been registered. So it is better to register WhatsApp Business API with a number that has never been registered for WhatsApp or WhatsApp Business.
Once the number is registered to a WhatsApp Business API, it cannot be migrated and donwgraded. If you want to use WhatsApp and WhatsApp Business, you must obtain new phone number.
For more detail information, you can refers to these link https://developers.facebook.com/docs/whatsapp/guides/phone-number/