Wouldn’t this require an amendment to the Constitution?
I'm not sure.
The Constitution says in Article I Section 8: "To exercise exclusive Legislation in all Cases whatsoever, over such District (not exceeding ten Miles square) as may, by Cession of particular States, and the acceptance of Congress, become the Seat of the Government of the United States..."
First, it says that the district is to be no more than 10 square miles, but it can be smaller.
However, secondly it says that the "cession" from the states AND acceptance of Congress must occur. That already has happened. I don't think Congress can now change its acceptance at a later date, because it was ceded by the states with an understanding in place at the time.
The article says that past precedence is already there to "retrocession" the unused land back to the original ceding states. It doesn't say that Congress can ask the original ceding states for permission to reuse the land for new current purposes (such as to grant it independent statehood).
Article IV Section 3 says: "New States may be admitted by the Congress into this Union; but no new States shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress."
In the case of converting DC into a state, Congress would first have to declare the extra space as no longer "the seat of government." At that point, Congress would no longer "exercise exclusive Legislation in all Cases whatsoever, over such" non-seat of government territory. Once that happens, Congress no longer has the authority to declare it a state; that land returns to the ceding states which must then each pass legislation allowing that land to be formed into a new state via Article IV.
-PJ