Signal for example, will warn you when the person you are talking to is using a new device.
As long as the user heeds the warning, it is an effective stop, and at the very least gives the user pause.
If the signal safety number changes, but the communication stays on track, as in, the context of the conversation is the same, it’s unlikely to be a problem. But if the safety number changes and the next message is asking for money, that is a very simple and easy to process situation.