There are two ways to answer that. One way is that if one was just going to set out to create a new convention, I think Professor Russell was suggesting there'd be some statement that was agreed to by all the leaders of the parties and you would need all four in order to do it, and that would be, in essence, the normal way of instantly creating a new convention, because all the major actors would have agreed.
There's a different sense in which a convention relating to Parliament may be created, and that is where the majority makes a motion and declares something to be the case. It could be on what does or does not constitute a matter of confidence, for example. In that case, in my view, it doesn't matter whether the government agrees or not. If the majority of the House says that this is or is not a matter of confidence, that is a definitive statement, and it changes the constitutional convention relating to confidence. It can do that because the essence of confidence is whatever the House says is confidence or not.
In terms of prorogation and so on, it's one of those situations where the House's opinion is in fact ultimately the most determinative. If a majority passes it with the dissent of the government, then I would say that the majority has still created a convention that is binding on the government.