Block chain itself is based on hashes, which are already quantum hard. QC will not break SHA256. However addresses are based on ECC crypto, which would no longer be secure. These would have to be replaced by another scheme like supersingular isogeny ECC or lattice based crypto. Old BTC addresses would need to have their coins moved out of them to new addresses or those coins could be stolen by anyone with a sufficiently advanced QC.
There's a lot of post-quantum crypto research underway.
Interestingly this would permit the recovery of "lost" coins, bringing more BTC into circulation. Any coins that weren't moved -- meaning nobody cared about them or wallet keys had been lost -- would get recovered and returned to circulation.
What I consider much more intriguing is how the NSA is apparently hoovering monstrous amounts of Internet data and storing it on disks. Much of that data is encrypted, but once QC becomes available they will be able to go back and decrypt almost all of today's Internet traffic. That will be... interesting. It would be kind of like a crypto "jubilee" in which all yesterday's secrets become known.
It's possible that bigger QCs will appear in the "black" world before they are publicly known, but in that case they're not likely to be used to steal bitcoin since doing so would indirectly reveal their existence. If too many BTC wallets get silently snarfed, people will suspect a break. Anyone with such a QC would be better off sitting and passively monitoring for as long as they can.
https://blog.ethereum.org/2015/12/24/understanding-serenity-...
There is also a new project called the Quantum Resistant Ledger. https://github.com/theQRL/qrl
So if the potential is there, and the science is there, I feel all blockchain technologies are at risk.
That said, the method of transaction signing would need to be reworked to be quantum safe. But not all Bitcoin would be able to be disappeared overnight.