Before you stop using a particular phone number or phone, you should migrate your WhatsApp account to the new number or new phone.
For a simple way to delete your old account number and migrate your data to a new number, use our Change Number feature.
Make sure your contacts delete your old phone number from their address books and input your new number. As it is a common practice for mobile providers to recycle numbers, you should expect that your former number will be reassigned.
Once you stop using your account, if the same number is used to activate a WhatsApp account on a new device after 45 days, your old account will be deleted completely.
If you are moving from one type of smartphone to another and preserving your number, you will be able to migrate your account data, such as subscription information and group participation. You will not be able to migrate your messages.
If you are moving to the same type of smartphone, such as an Android to another Android, then you may have an option to migrate your messages. Read more here.
Before you give away your smartphone, make sure to wipe the phone of all your data. Remember also to wipe the SD card, if your phone has one. This ensures that none of your private data, such as your WhatsApp messages, will fall into someone else's hands.
WhatsApp Support Team