On the new idea, the nostr master key would generate a new key for every device. The user wouldnt be able to export it. When you change phones, people would be forced to rotate their keys for the DMs, not for the rest of nostr. Their profile stays the same, but messages go somewhere else.

If we then add private groups (nostr doesn't have this yet) we could add all device keys in the same "group" so that all your DMs can be seen by all your devices.