I’m not sure you can form a state from within a state. If you look at Article 4, Section 3 where it states “but no new States shall be formed or erected within the Jurisdiction of any other State”. That phrase is bordered by semicolons, meaning it’s a separate thought from the one listed behind it where it says “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”. That means, to me, that the part where the states Legislatures (notate plural) and Congress are mentioned is only if the “new state” is created out of parts of more than one state.
Just my opinion but those semicolons are there for a reason.
But the precedent is already there, Kentucky was formed from Virginia, Maine was the Maine District of Massachussets, West Virginia was formed from Virginia (with the approval of a rump Unionist Virginia Legislature). You can carve up a State, as long as the State’s legislature and the Congress approve.