A change from ECDSA-based addresses to quantum-safe addresses would be no small fork, and would potentially require disabling active addresses for a period of time while a fork was implemented, regardless of the specific cryptocurrency. This could have significant deleterious effects on a cryptocurrency-powered blockchain network, and, as we have experienced in creating our own blockchain, could also require the changing of significant sections of the cryptocurrency’s code to accommodate the new security features, drawing into question the feasibility of implementation.
Additionally, one cannot always (or, one could argue, ever) predict when and where technological innovation will rapidly progress. This is especially true of emergent technology, and both blockchain and Quantum Computers would qualify as such. There is potential for an unforeseen/unpublicized advance in Quantum Computing leading to an attack on a cryptocurrency network, and the market-wide realization of the sudden vulnerability of cryptocurrencies that are based on ECDSA signature methods. This would likely cause a “run on the banks” scenario and crash the value of many-if-not-most cryptocurrencies that were secured by ECDSA.
Finally, even if the signature scheme of a particular blockchain platform can be updated to something with Post-Quantum security, there will never be a 100% adoption rate among the total set of wallets. There will always be old wallets which are no longer accessible (due to lost keys) and which cannot be updated, and going forward there will be an ever-larger set of non-technical users unable/unwilling to take necessary steps in a timely fashion should a sudden QC threat emerge. The coins in those old wallets could be vulnerable if the associated account ever broadcast a transaction, and an attacker with a sufficiently powerful quantum computer could accumulate a non-trivial percentage of a coin’s circulating supply and use it to manipulate the price of that cryptocurrency. It is not guaranteed that an attack of this sort would be immediately noticeable.See more FAQ's