It is very easy to Change WhatsApp phone number. I think If you are changing your phone number to new one, You will be delete your using WhatsApp and install new one, But actually don't want to unistall your using WhatsApp, You can change the phone number, Just follow these steps for changing your WhatsApp phone number to new one.
Also read : How to hide WhatsApp photos from phone gallery
* At first delete your old number WhatsApp account
* And then want to migrate your account to your new number
* Then Open the stings > Account > Change number > Ad your phone number.
Note: If you are using iPhone follow same as said above