As far as secession Cozie, there's no Constitutional 'mechanism' for it. The 'common wisdom' is that a State cannot legally secede. A State could say they've seceded, but if they tried to seal borders or take over Federal facilities and kick out Federal workers, I'm sure the military would be sent in to 'secure things'. This is what happened at Ft Sumpter. The Confederacy shelled the fort and Lincoln sent in Federal reinforcements to protect it. Hey Presto, the Civil War (or as it's called down South "The War of the Northern Aggression" 😆). Honestly, I don't think any State wants to test the theory that they can secede. But who knows, crazier things have happened (ie Scrotus).
As far as 'major changes', I think hope that a lot of what Scrotus has done can be 'undone' pretty rapidly. Anything he's done by EO can be cancelled by another EO by the new POTUS. Cabinet members and other political appointees can be fired in a trice as they 'serve at the pleasure of the POTUS'. I've seen my (former) agency reverse engines so fast it made our heads spin when a new administration came in. The new POTUS just has to put people in with the knowledge of how to make those changes and make them rapidly. And that means savvy policy people. Not one's 'besties', sycophants, or someone they want to 'do business with'.
Laws that have been changed (like the new tax law & some changes to the ACA) take longer because they have to go through Congress. And if the House or Senate majority is from the other party, they may not get changed at all.
DH and I were actually talking about this the other night. We can see a lot of environmental changes and immigration changes (for the better) once new Agency heads are in place. Right now we have Scrotus' oil and energy butt-buddies running the Interior Dept and no one really running ICE as there's only an Acting Director.
But I think we can expect a shit-ton of shenanigans (especially at Interior) before the end comes for the Scrotus Regime. It's already starting. Scrotus et al are already getting 'antsy' and making grabs in plain site that before they'd try to hide (like the G7/Doral thingy). They're like thieves who have seen the headlights turn into the driveway so they're grabbing what they can before skedaddling out the back door and over the back fence.