I think you'd just make the sockets and plugs keyed. A 120v only plug can only plug in a 120v outlet. A dual voltage plug can plug into either socket. Bonus poi

I think you'd just make the sockets and plugs keyed. A 120v only plug can only plug in a 120v outlet. A dual voltage plug can plug into either socket. Bonus points for providing some simple signal that indicates the voltage for dumb appliances like hair dryers. The other option would just be to require all appliances to be dual voltage. This would add several dollars to many devices but they cost would go down over time. It's academic anyway, a switchover isn't happening any time soon. The only way I can see it happening is if we switch to a system similar to EV charging where all connections are negotiated. Nothing is hot until required and anything operating over current can be immediately identified and disabled. Appliances could even limit their consumption to a lower setting if insufficient capacity was available on their circuit. Such a change could occur due to future regulation and hopefully at a time where the cost of the technology has gone down significantly.