Constitutional scholars are divided as to whether such a convention would be limited to the one subject for which it was summoned or whether delegates could expand the agenda as they might see fit. Opponents of a convention assert "There is no consensus as to whether or not the convention would have the power to simply disavow the Constitution altogether and propose replacing it with an entirely new document, and that it is worth noting that such an outcome would be similar to what happened when the Convention of 1787 ended up writing the current Constitution. That convention was called for the purpose of revising the earlier Articles of Confederation, but it chose instead to abandon the articles in favor of a completely new document." The 1787 convention was authorized by the Articles of Confederation, by an act of Congress, and by votes of the states; and that under the Act of Congress, any action taken by the 1787 Convention was invalid unless ratified by both a vote of Congress and the states.
For some reason, this made me think of The Second Vatican Council. Scary.
I tend to believe it is meant to be the latter. There is already a mechanism in place for amending the existing Constitution. A Constitutional convention could write a new Constitution from scratch.
It would be essentially a re-boot of our system.