They wouldn't even be able to.
WA is end-to-end encrypted.
WA is end-to-end encrypted.
a) that would involve changing the code base b) more privacy minded users would just delete the messages or not update.
Think about it for a second: you can re-establish your WA account on a new device using only the SIM card from your old device. SIM cards don't have a storage area for random applications' encryption keys, and even if they did, a SIM card cannot count as "end-to-end" anymore. Same goes for whatever mobile cloud platform those backups might be stored on. And you'd hope Apple or Google aren't happily sending off your cloud decryption keys to any app that wants them. Though maybe they are?
In any case, as soon as you start using WhatsApp on a new device, users in the chats you participate in will receive a message informing them that your encryption keys have changed.