The Change Number feature allows you to change the phone number associated with your WhatsApp account on the same phone. This feature is intended to be used before you verify your new number.
This process will:
You can also change your WhatsApp number on your device by simply inserting your new SIM into your phone and opening WhatsApp. WhatsApp should prompt you to re-verify your new phone number.
Note: If you do not use the Change Number feature, your account information, groups, settings and messages will not be migrated.
Learn more about changing your number on: Android | iPhone | Windows Phone | BlackBerry | Nokia S60 | BlackBerry 10
Cheers,
WhatsApp Support Team