As quantum computing approaches, how quickly blockchain networks can adapt is emerging as a key challenge. Quantum computers could theoretically break digital signatures and encrypted communications, posing a serious threat to blockchain security. Post-quantum cryptography is already being adopted, and the U.S. National Institute of Standards and Technology has directed federal systems to complete a quantum-security transition by 2035. Permissioned blockchains may respond faster, while public chains such as bitcoin and ethereum must navigate community-driven proposal processes and user adoption.