"Quantum Computing will not break Xtrabytes"
http://xtrabytes.today/technology/security/quantum-computing/
Another interesting read. I would imagine that if someone could break into the system they would do it in one big attack. It could be updated but the coins would be gone. It's way down the road most likely but if it did happen i wondering how the XBY team would respond.
I think I remember borzalom saying that the security algo can be changed, so if in the future the current one becomes an issue due to quantum computing (way in the future) then the plan is to change it to newer developments before such an attack can take place.