Google says DC was established in 1790 as decreed in COTUS. If that is correct, it sounds like a constitutional amendment would be the qualifying event to make it a state.
All they have to do is split the district. Leave the 10% of it with all the federal buildings as the seat of government and 90% as a new state. They have it all figured out. It would be our only purely welfare state...until they admit Puerto Rico.
