Anyone who uses WhatsApp will know how easy it is to use. That is because they have made it as user friendly as possible, but with ease of use comes a trade-off of security and privacy.
You either have easy to use or secure; it is a sliding scale between the two!
The way WhatsApp works, is that it ties your account to your phone number. Your device and their servers will have a set of encryption keys that make it secure to communicate with them. Now the problem comes when you change your phone number. What happens to your old phone number? Well someone else will likely get it because phone providers recycle numbers. Can you see where this is going? The new owner of your phone number will now have access to at least some of your messages when they sign in to WhatsApp with their new phone number (your old number) or vice versa. It is reported that they do not necessarily have full access to everything but they can have enough access to find out a lot about you.