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.
ANY---I repeat---ANY gathering of national delegates empowered to amend ANY part of the Constitution will begin by amending their own instructions, and then INVALIDATING the ratification process. (This is precisely what the Articles congress did.)
You don't think it will be a Concon, but I guarantee you, THEY WILL and they will treat it that way.