If you must switch to a new phone, back up your chats to Google Drive (Android) or iCloud (iPhone) to ensure you don't lose your message history.

To continue using WhatsApp reliably, your device must meet these minimum software standards: OS 4.1 or newer. iPhone: iOS 12 or newer. Affected Devices

Huawei Ascend G740/Mate/D2, HTC Desire 500, and several ZTE Grand models. What Should You Do?