On Oct 7, 2015, at 9:02 AM, Eric Lombrozo <elombrozo@gmail.com> wrote:

> But a real hashpower supermajority would make such attacks hard to pull off in practice.

If you had a 99% hashpower supermajority on the new version, an attacker would still be able to perform this attack once per day. Since the attacker is creating a transaction which is invalid according to new clients, it will just sit around in old clients' mempool until one of them mines a block.