WhatsApp offers a migration path from the WhatsApp for Business App to the WhatsApp for Business API.
First of you need to get your display name approved. Submit the phone number to us. You can find instructions on the submission process here. It is required to WhatsApp for Business account associated with that phone number. While you are migrating your number all incoming messages will be queued by WhatsApp servers and delivered after a successful migration.
You must now complete number registration as described here.
If that phone number has not been used on WhatsApp Business API, you can use that phone number. Follow the migration steps outlined here to reuse that phone number.
Deleting the WhatsApp account on your Android or iPhone will cause any incoming messages to be queued by the WhatsApp servers and then subsequently delivered upon successful registration.
There is currently no option to import previous conversations.
No. At present, number migration from a BSP to another BSP is NOT possible.
Official Business Account approval is only available for selected WhatsApp for Business API clients. It's not affected by the migration. Once the number has been upgraded to the WhatsApp for Business API it is possible to apply for the OBA status.