I think what you’re getting at is “if a new opcode is added, what if it has a bug” …. but it doesn’t matter. There’s no such thing as “older Y-immune software” once the fork goes into effect. You MUST upgrade. Please re-read my article, part of it is addressing this idea that people can pick and choose which features they accept. There’s no such thing. Once the majority takes the fork whether it is hard or soft is irrelevant — you will upgrade whether you like it or not, as the alternative is losing the security you are running the program to get in the first place.