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.