This trick will teach you how to migrate your whatsapp account to a new number. The steps involved are very simple and easy to understand. Just follow the steps.
But before starting the process it should be noted that Changing your phone number will migrate your account info, groups and setting. And you will need to confirm that you are able to recieve SMS or call at your new number.
For Android Users
Tap on Menu > Settings > Account > Change Number and enter the old phone number in the first box, followed by the new number in the second one.
For iPhone Users
Go to Settings > Account > Change Number, Enter your old phone number in the first box, and your new phone number in the second box