We are aware of the upcoming mobile number changes in Brazil and we have an automated system to sync your account to the new number - you don't need to do anything!
The new round of phone number changes in Brazil happening on 05/31/2015 will affect the following area codes: 81, 82, 83, 84, 85, 86, 87, 88, and 89.
We are prepared for this change and we are making all necessary adjustments on our end so that you can continue to use WhatsApp seamlessly by sync'ing your new account to your old one. This means you will keep all your existing chats and media (photos, videos, voice messages and audio). All messages sent to both your new or old phone number will be delivered to you.
However, in some cases you might see two different chat entries for the same contact, one for the old number and one for the new. Unfortunately, there isn't a process do merge these chats at this time.
WhatsApp Support Team