Back up your chats in whatsapp 

To make sure you don't lose any of your chats if you lose your device or you switch devices, make sure you back up your chats to iCloud or Google Drive. 

Backup your chats in whatsapp

Unfortunately if you are switching between iOS and Android and vice versa, there is no simple way to move your chats over so be prepared to lose them. Also, be aware that archived chats are no longer encrypted.

iOS & Android: Settings > Chats > Chat Backup > Back Up Now.