Before you stop using a particular phone number, you should migrate your WhatsApp account to the new number. You can easily do this by using WhatsApp’s Change Number feature.
Make sure that your new number is able to receive SMS verifications or calls and has an active data connection. Make sure that your old number is currently the verifired number tied to your account.
Make sure your friends and family delete your old phone number from their phone's address book and input your new number. It is common practice for mobile providers to recycle numbers, whereby your old number will be reassigned. This may lead to your friends and family messaging the wrong person after your old number has been reassigned.
You do not need to manually delete your old account. After successfully using the Change Number feature, your old account will automatically be deleted.