WhatsApp uses the phone number as a label for your service and connects you to other users independently of it via the net, it doesn't use the telecoms switching system to reach your phone. Apart from any initial validation it might use it has no further use for your link to the telephone number you use. As a WhatsApp user, at least on my android, the number enables the system to find other contacts who have the app. So, if you change your phone number but not the one used for WhatsApp there is the possibility, arguably remote, that you will be linked to a future user of that number. (Imagine LE searching your old number because of something the new phone user had done and finding your WhatsApp.) If the new user wanted to use the app, in the best case (from your perspective) they would be unable to do so, and in the worst case they may be able to override your WhatsApp account to create their own.