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...
Exactly, NIP-05 should be considered as an alias, not as complete verification.
The identity relies in the keypair, not in the DNS domain.
The identity relies in the keypair, not in the DNS domain.