He can sign with his old privkey a statement to verify his new keypair, as he has not lost control of the old key.
Much better than using NIP-05, as that needs trust in lots of systems (DNS hierarchy, webserver security, etc), imo.
Discussion
#[3]
Why trust any nip-05 then? It’s a much more visible and user friendly instructor. It would be an appropriate measure to take.
Exactly, NIP-05 should be considered as an alias, not as complete verification.
The identity relies in the keypair, not in the DNS domain.