The following platforms are no longer supported as of the beginning of 2017 as mentioned by whatsapp:
- Android versions older than 2.3.3
- Windows Phone 7
- iPhone 3GS/iOS 6
On June 30, 2017, whatsapp will no longer support the following platforms:
- BlackBerry OS and BlackBerry 10
- Nokia S40
- Nokia Symbian S60
"These platforms don't offer the kind of capabilities whatsapp team need to expand our app's features in the future. If you use one of these affected mobile devices, whatsapp team recommend upgrading to a newer OS version, or to a newer Android running OS 2.3.3+, iPhone running iOS 7+, or Windows Phone 8+ so that you can continue using WhatsApp".
Once you have one of these devices, simply install WhatsApp and verify your phone number on the new device. Keep in mind that WhatsApp can only be activated with one phone number on one device at a time.
There is currently no option to transfer your chat history between platforms. However, whatsapp update will provide the option to send your chat history attached to an email.
Credits-WhatsApp Support Team