So the old Union of 14 years was dissolved in favor of the new less restrictive Union it would seem to me. The basis of that New Union was the Constitution. Without that Constitution, because of the death of the old alliences among the several states, there would have been no union. In fact there seems to have been a concerted effort to entice certain states to join that Union written into that Constitution.
I cited the much discussed "Slavery Clause". While most say that this clause was placed in the Constitution to entice the Southern States to ratify that Constitution, I believe that it was placed there more so to entice the states of New York and New Jersey who depended on slaves the work their docks and wharfs. It would not have mattered to a hill of beans if the 4 southern slave states failed to ratify the Constitution if the 9 northern states still ratified the Constitution. But if New York had not ratified the Constitution, and it was one of the last to do so, then the New Union would not have survived being split into two parts by New York even if the other 12 states had ratified it.
If it was necessary to "Tailor" the Constitution to special interests, either the southern states, or New York and New Jersey, then it was an effort to forge a Union from seperate states individual interest. So if it was necessary to forge a new Union then the sprite of unity to form a Union was very weak if it existed at all. So therefore there was no Union, no sprite of co-operation between these states, prior to the Constitution once the decission to abolish the old Union under the Articles of Confederation and write a new Constitution was made. In fact that sprite seemed to be very strained to even make necessary the dissolution of the Articles of Confederation and the writting of that Constitution with much debate. It was the desire of those states to be more in charge of their own fate that lead to that decision. So how was there a "Union of one purpose" there? This New Constitution and the New Nation was a gamble that anyone of the state let alone 9 of them would join it.
Yes, the Constitution was flexible in it's form because of the foresight of adding the Bill of Rights and an amending process, but that is what has maintained the Union. Yes, it has changed the Union as each amendment has been added. But not because of the Constitution. An Amendment can become Law even when it is anticonstitution or unwise. Such as the 16th Amendment (Income Tax), 17th Amendment (direct election of Senators) and the 18th Amendment (Prohibition).