A future version of WhatsApp will let users migrate their chats data across platforms. The feature is currently available in a beta version of the app and only allows iOS to Android transfers. There is no word as to when it will be available on the stable version of WhatsApp, though.
https://www.notebookcheck.net/WhatsApp-will-soon-allow-users-to-transfer-chats-between-Android-and-iOS-devices.530988.0.html