Coinbase has published a detailed technical paper arguing that Bitcoin's migration to quantum-resistant cryptography must begin years before any Q-Day. Q-Day is the day a quantum computer can break the cryptography behind Bitcoin wallets. The exchange's case is short: the work is multi-year, so it has to start now.
Block space is the first hard constraint. Post-quantum signatures are larger than the ones Bitcoin uses today. Larger signatures mean more data per transaction, and less room in each block for everything else. That shifts fees and throughput before a migration even settles on a winner.
Custody is the second. Moving funds is not a one-time swap for a holder. Exchanges, banks and hardware wallet makers all need to generate new key types, store them and support both old and new signature schemes while the network transitions.
Coinbase, which custodies a large share of institutional Bitcoin, frames this as an engineering programme that needs lead time, not a patch.
Dormant coins are the third and the hardest part. Coins sitting in addresses whose public keys are already exposed cannot all migrate on their own. Some belong to people who lost their keys.
Some belong to people who are long gone. Deciding their fate is a network-level governance question, and Coinbase's paper does not settle it. It argues the engineering should not be held back waiting for that answer.
The two tracks can run separately. The first is technical: pick a post-quantum signature scheme, test it, and ship wallet support. The second is governance: what happens to coins that never move. Coinbase's point is that the first still takes years. Real progress starts when it shows up as code in review.
The sticking point to watch is whether a post-quantum proposal actually enters Bitcoin's formal review track. That is the first observable sign the multi-year clock has started.