It's in their ratification documents and they were admitted to the union, so they weren't old "no."
In toto and forever, bub.
Right from the words of Madison himself when asked if a State could secede:
==================
To Alexander Hamilton
[July 20, 1788]
N. York Sunday Evening
Yours of yesterday is this instant come to hand & I have but a few minutes to answer it. I am sorry that your situation obliges you to listen to propositions of the nature you describe.
My opinion is that
a reservation of a right to withdraw if amendments be not decided on under the form of the Constitution within a certain time, is a conditional ratification, that it
does not make N. York a member of the New Union, and consequently that she could not be received on that plan.
Compacts must be reciprocal, this
principle would not in such a case be preserved.
The Constitution requires an adoption in toto, and forever. It has been so adopted by the other
States. An adoption for a limited time would be as defective as an adoption
of some of the articles only. In short any condition whatever must viciate the ratification. What the New Congress by virtue of the power to admit new
States, may be able & disposed to do in such case, I do not enquire as I suppose that is not the material point at present. I have not a moment to add more than my fervent wishes for your success & happiness.
This idea of reserving right to withdraw was started at Richmd. & considered as a conditional ratification which was itself considered as worse than a rejection.
http://www.constitution.org/jm/17880720_hamilton.txt
But if you think unilateral secession is allowed -
Go ahead. Try it. See what happens
... if you secede, you'd better succeed.