[likelynew]

> Fortunately for 1) (I think) it currently takes 2^512 (?) operations to break the private/public algorithm which is unfeasible to brute-force on normal hardware but a QC brings it down to 2^128

Where have you got that info? Quantum computers can break ECDSA in polynomial function of 512.

> the blockchain could be changed overnight to use a new keying algorithm.

How?