Yes, this happens automatically in whatsapp. In Signal, you will be prompted to confirm the new key.
Still, _it is possible to trigger this process for a third party_, and then to MITM the key exchange.
Still, _it is possible to trigger this process for a third party_, and then to MITM the key exchange.
I'm not exactly comfortable with this now - it seems like someone may be able to spam messages to hide the key change.