First of all you need to understand how WhatsApp identification really works.
On the first installation of the app, WhatsApp will ask you your phone number. The app will use this number to identify you for the first time thru a security code sent by text.
This phone number will be like your username with WhatsApp and will never change until you decide or you’re not the owner of the phone number anymore. WhatsApp never directly use your actual phone number to send messages or place calls, it just serve to identify you.
Inserting a new sim card
When you insert a new activated sim card, a phone number is usually attached to it, except data only sim cards.
WhatsApp will detect a new phone number and prompt you if you wish to change your number.
If you still want to be contacted on your current phone number, simply answer “No”.