WhatsApp the easy to use chat program from Facebook has been found to have a major security flaw. WhatsApp uses the user’s phone number for both account name and delivery of the messages. This is a great and secure way to do this as it means that the messages will only be delivered to the user they are intended for. Until that is the user, for whatever reason, gets a new number. If the number has been used previously by a WhatsApp user then it is possible, and has happened, for the new owner of the number to get some access to the old users WhatsApp messages and groups. This is because of the way that WhatsApp uses the phone number for authentication to the account. So your number that you don’t have anymore may still have access to your old messages.
WhatsApp say that their help section does have help to prevent this from happening, just remember to read it before you change your number or as soon as possible after, the sooner the better because phone numbers get recycled and so someone will have access to your messages and friends if you don’t.