Are you of the opinion that the US Constitution was unlawfully brought into being?
If you choose to stand on the legal pillar that says a COS cannot make dramatic changes, then yes, the current constitution is not legal because the delegates that went in 1787 did not have the power to draft a new constitution.
That said, I am of the opinion that anything can come out of a convention which is based upon our known history.
To that end, I do not wish to see any kind of convention where delegates gather believing they are supported by the power of the people.