OK, here is somewhat of a summary.
There are those who say the Articles were replaced... they are wrong.
Congress under the new Constitution authorized the collection and printing of all the laws. These are printed in the United States Statutes at Large.
The first volume was published about 1847 and includes all the laws passed by the Congressional sessions up till then and the basic laws of the United States.
Pages 1-3 have the Declaration of Independence.
Pages 10-22 have the Constitution of the United States.
Pages 4-9 have the Articles of Confederation.
The Constitution created a “more perfect union”, a POLITICAL entity that would come to be called “The United States”.
The split - the actual division - was investigated and defined in the early 1900’s in a series of decisions called the Insular cases that came before the Supreme Court.
Part of those decisions answered the question What is “The United States”.
They ruled there are three definitions. “United States” can mean:
The original Union formed of 13 (now 50) sovereign states
The national government, the ten sq miles over which Congress exercises exclusive legislative jurisdiction
The geographical United States, as it would be used by a person say in Europe who says “I’m going to the United States on holiday”
Now. These might seem trivial distinctions. Far from it. The insular decisions were questions basically about things going on in the territories, not the states. And the questions were can Congress legislate in an obviously unconstitutional manner for things happening in the territories.
The Supreme Court said... yes.
Congress IS NOT CONSTRAINED by the Constitution in legislating for things that happen in DC or the territories because THEY ARE NOT A PARTY OR PART OF THE UNION. This is why it takes legislation or Supreme Court decisions to “incorporate” various things to be effective in DC, like the Heller decision. Even today I believe you can get arrested in DC for distributing handbills on the street.
Further proof comes in the citizenship decisions, where it was ruled that it is not necessary for a person to be a citizen of the United States to be a Citizen of a state.
We still have the Union... barely...
Interesting points. One quibble--The union was already called the United States under the Articles of Confederation.
Article I. The Stile of this Confederacy shall be "The United States of America."
The whole perpetual union/more perfect union debates are pretty interesting. You can't get through the secession period leading to the civil war without encountering it. The Webster-Hayne debate is, iirc, the definitive presentation of the arguments.
The distinction between states and territories was also a big factor in the controversy surrounding the expansion of slavery. The anti-slavery forces used the distinction you cite to justify using federal power to prevent expansion of slavery wherever possible. That, of course, led eventually to civil war.