A hard fork implies a difference in consensus rules, and what do you propose that difference be?

Existing wallets need to actively commit to some PQ signature mechanism, prior to Q-day.

Even if Q-day means there is a way to deterministically retrieve any private key from a public key (is that what it means? or is the blast radius of q-day contained? This is a bit above my level of cryptography), I’m sure we could come up with something to minimize the damage. In the worst case, it might involve a claim process with an authority or consensus mechanism to prove who the rightful owner of the funds is and revert the unauthorized transactions on the new chain.

Yes, this is not ideal! But if the wallet conversion requires active participation, preemptive measures are also not ideal.

	▲	Retr0id 12 hours ago \| parent \| next [-]
		> Q-day means there is a way to deterministically retrieve any private key from a public key That's exactly what it means. (Note also that under ECDSA you can retrieve a public key from a valid signature). How do you prove anything, after the key material is compromised?
	▲	glerk 12 hours ago \| parent \| prev [-]
		> How do you prove anything, after the key material is compromised? It’s a blockchain, so the simplest would be chain of custody until the chain points undeniably at you. This is not a pure cryptographic device, some social intervention might be needed here.