A hard-fork is a situation where non-upgraded nodes reject a block mined and relayed by upgraded nodes. This creates a fork that cannot heal regardless of what follows.
This proposal is not a hard-fork, because the non-upgraded node *will heal* if the attack has less than 1/2 of the original-POW power in the long term.
The cost of such an attack is the cost of a normal "51%" attack, multiplied by the fractional weight of the original POW (e.g. 0.75 or 0.5).
So rather than saying this is a hard-fork, I would say that this is a soft-fork with reduced security for non-upgraded nodes. I would also say that the reduction in security is proportional to the reduction in weight of the original POW at the time of attack.
As mentioned before, the original-POW weight starts at 1.0 and is reduced over a long period of time. I would set up the transition curve so that all nodes upgrade by the time the weight is, say, 0.75. In reality, nodes protecting high economic value would upgrade early.
If a block that would be discarded under previous rules becomes accepted after a rule addition, there is no reason to not simply call the new rule a hard fork. IOW it's perfectly rational to consider a weaker block as "invalid" relative to the strong chain. As such I don't see any reason to qualify the term, it's a hard fork. But Peter's observation (the specific behavior) is ultimately what matters.
+1 to all of Peter Todd's comments
bitcoin-dev mailing list
bitcoin-dev@lists.linuxfoundation.org
https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev