Delegates to the convention will have detailed commissions that describe the extent of their authority.
Nope.
The difference is that there wasn't a Constitution in place the first time, and there is one now.
The justification for the Convention to propose amendments is Article V. There was no Article V before.
If you think the Convention to propose amendments can just scrap the Constitution outright, then you must believe that they won't need to have any states ratify it, since ratification is in the Article V that was just scrapped?
Furthermore, how do you address the fact that the states are still sovereign governments? Do you believe that the legislatures of the several states would just sit back and let a Convention to propose amendments tell them that the Constitution has been voided and they are now in charge?
Since the states were sovereign governments that preexisted the Constitution, wouldn't scrapping the Constitution just revert the states back to individual sovereign governments? If the Convention for proposing amendments decides to ignore that Constitutional boundary on their powers and declare a new Constitution is now in force, what would behold the states to this new Constitution?
Even the first Constitution was submitted to the states for ratification. Are you suggesting that a new Constitution to replace the existing Constitution would not have ratification processes as well, and would be imposed on the sovereign states by Convention fiat?
-PJ