There are numerous options for quantum-resistant algorithms. Existing cryptographic currencies can fork to whatever seems most suitable. This wouldn't be a trivial change, but it can certainly be done. https://en.wikipedia.org/wiki/Post-quantum_cryptography

There is also no such thing as 'unlimited' computing power with regard to proof of work. Difficulty simply adjusts, and the cost of a network attack will always be related to the rewards of mining.