They'd be free to leave. The instant TX secedes the price of property in the state will go up (from all those trying to move there), so they will suffer no harm financially . . . at least til they move to the slave states. A very tentative opinion assuming no outbreak of war.
And if they chose to stay and defend their homes and their communities would you forcibly evict them? Put them in cattle-cars? Imprison them?
“They’d be free to leave.”
As is anyone, individually or as part of a group, who currently resides within the US and does not consent to their present local, State or National government.
I infer from your answer that you think such people have no right to secede from their State at any level (county, city, etc.) nor to alter or abolish their government and institute a new one.