There are 2 options:
1) Connect a new number. In any case you need to re-setup all your robots when switching to WABA. So, it will be easier to connect a new number for testing.
We recommend connecting a new number and leave the old one in your phone to save chats.
2) Use the old number. We do not recommend connecting the old number, you can lose chats if they are not in CRM.
⚙️ To connect the old number your need to delete it from WhatsApp mobile app (Settings → Accounts → Delete).
❗️When connecting WABA from any number you’ll need a new license.