There would be a period when both systems are active. It will likely require more bytes to log a quantum-secure transaction. With no other protocol changes, t

There would be a period when both systems are active. It will likely require more bytes to log a quantum-secure transaction. With no other protocol changes, this would incentivize people to keep using ECDSA keys. And the increase in transaction size, would cause reduced transactions per second. No inherent problem. The protocol could have a built-in discount. Maybe the block cap should be increased to compensate. Questions like that are not so easy to get consensus on.