Did you forget about West Virginia? They split from Virginia. The trick is the state legislature has to approve the split and so does the US congress.
They made an exception to form WV.
None the less(it was also done during the secession period),it is a precedent, one in this country, the only one that counts. Splitting off of a state is perfectly constitutional as long as the procedures are followed. As I said, Colorado state legislature has to approve the split, ditto the US legislature, then viola, the deed would be done, a new state, Northern Colorado.
I live in CA but love Colorado, I have been hunting there many times over the years. It is sad to see it slipping into the same liberal quicksand that has dealt a death blow to CA.
The federal government won the approval of the rump legislature (representing the parts of Virginia under Union occupation) to go ahead with the admission of West Virginia as a separate state. I think the people behind the formation of West Virginia were mostly from the northern counties (settled from Pennsylvania) whereas many of the southern counties of what is now WV (settled from the older parts of Virginia) still were loyal to Virginia and supportive of the Confederacy. I think there were a lot of counties in southern WV which did not participate in the 1864 federal election. I had relatives in the southern part of WV who served in Confederate units.
The Old Dominion was not renamed East Virginia...although I think the Perfesser in "Shoe" thinks he lives there.