I would think that they were no longer members of any union. I fail to see how the union was perpetual, when 9 members abandoned (secede) from it, forming a new, separate union no consisting of the 13 former members. Additionally, it the AoC still were in force, where is the Congressional act ending the Articles, confirmed by each member?
James Madison, in Federalist No. 43 held that the union was no more:
'What are these principles? Do they require that, in the establishment of the Constitution, the States should be regarded as distinct and independent sovereigns? They are so regarded by the Constitution proposed.'And that the Articles were abandoned:
In one particular it is admitted that the convention have departed from the tenor of their commission. Instead of reporting a plan requiring the confirmation OF THE LEGISLATURES OF ALL THE STATES, they have reported a plan which is to be confirmed by the PEOPLE, and may be carried into effect by NINE STATES ONLY. It is worthy of remark that this objection, though the most plausible, has been the least urged in the publications which have swarmed against the convention. The forbearance can only have proceeded from an irresistible conviction of the absurdity of subjecting the fate of twelve States to the perverseness or corruption of a thirteenth.In Federalist No. 41 he asks two questions: '1. On what principle the Confederation, which stands in the solemn form of a compact among the States, can be superseded without the unanimous consent of the parties to it? 2. What relation is to subsist between the nine or more States ratifying the Constitution, and the remaining few who do not become parties to it?'
His reply to 1: 'The first question is answered at once by recurring to the absolute necessity of the case; to the great principle of self-preservation; to the transcendent law of nature and of nature's God, which declares that the safety and happiness of society are the objects at which all political institutions aim, and to which all such institutions must be sacrificed.' In other words, the sovereigns of each state can unmake (secede) from the Articles WITHOUT the consent of any other state.
His reply to 2: 'no political relation can subsist between the assenting and dissenting States.'
Clause 1: No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
Clause 2: No State shall, without the Consent of the Congress, lay any Imposts or Duties on Imports or Exports, except what may be absolutely necessary for executing it's inspection Laws: and the net Produce of all Duties and Imposts, laid by any State on Imports or Exports, shall be for the Use of the Treasury of the United States; and all such Laws shall be subject to the Revision and Controul of the Congress.
Clause 3: No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.