Doesn't work like that. See the original US Constitution. When these groups get together, the first thing they do is vote to ditch the rules. When the US Articles convention met, they didn't even have a means to ratify the resulting document.
It is just silly to think that ANY existing rules---even to "be approved by 3/4 of the legislatures"---would BIND any Concon of this type.
It is just silly to think that ANY existing rules---even to "be approved by 3/4 of the legislatures"---would BIND any Concon of this type. That's just it, it is not a constitutional convention, but a convention for proposing amendments — it's an important point.