Although this Article states that the union cannot be violated by any state, it does allow for amendment to the Articles through agreement in Congress and approval by each of the state legislatures. The convention of all states, except for Rhode Island, in Philadelphia in May 1787, met under this guise to "amend" the Articles, not replace them. When the new and radically different document was adopted by this convention, it had to be ratified by all of the states before it took effect.
It is questionable whether this transition between the Articles of Confederation and the Constitution was legal at all. The Articles stated that Congress had to agree to amendments, and the convention was not Congress. However, given the weakness of the Congress and the strength of the states, it would have been futile to try to stop it. Plus, Congress, more than any other group, probably understood the difficulties inherent in a powerless central government, and therefore welcomed the chance for increased powers after being so long held captive by the supreme sovereignty of the states.
The transition from Articles to constitution was somewhat in the spirit of Thomas Jefferson's Declaration of Independence, which stated that a people have a right to overthrow a government that does not protect its fundamental rights. This time, however, the people did not demand a more democratic government that put sovereignty solely in the state, but clamored for a stronger central government that had a better chance of protecting their property from the chaos of too many competing state governments.