Not true. The Constitution was unanimously ratified. Or was there some other illegality you were speaking of? Is our current Constitution illegal and illegitimate?
When the states were invited to send representatives to consider alterations, the resulting convention quickly discarded the notion of drafting amendments to the Articles and instead, and in secret, set to writing an entirely new constitution, essentially creating a new form of government.
When the Constitution was submitted in 1787, Congress initially debated censoring the delegates for exceeding their authority, among other ways by ignoring the Article's requirement that alterations be approved by every state. The notion of censure was soon dropped, and the proposed constitution was sent to the states complete with Article VII, which required only nine of the thirteen states for ratification. The Constitution also specified state ratification conventions as opposed to ratification by state legislatures as required by the Articles, as the Constitution's drafters feared that state legislatures would paralyze the process with numerous amendments.
After Delaware became the first state to ratify, other states started falling in line, and New Hampshire became the ninth in June of 1788. There was considerable fear at the time that the union formed by New Hampshire's ratification would be unworkable, as two key populous states, New York and Virginia, were not yet on board. Those two states did after bitter debate eventually ratify, as well as the remaining two (North Carolina and Rhode Island). Rhode Island, the last, was threatened with being treated as a foreign nation by the new United States, and ratified (by two votes) on May 29, 1790.
Fascinating times, those must have been.