Why would SHA-256 be vulnerable to quantum computing? Grover's algorithm only gives a square-root speedup for reversing hash functions, and that's in the best-case scenario where there are no engineering issues (e.g. the need for error correction) which could create polynomial overhead. That still leaves you with 128 bits of security.
Quantum computing could certainly break elliptic curve cryptography, but my understanding is that this only becomes an issue if you publish your public keys (e.g. if you use your key to digitally sign a message) instead of just publishing their hash, which is not standard practice in Bitcoin (for exactly this reason).
