I think there is a good chance some ersatz agreement will be struck that replicates many of the aspects of being in the EU so the change will be minimal.
But then, maybe not.
Europe is a scary place. Falling native populations, hostile muslim fifth column populations, mass-immigration, economic stagnation, rising political instability…
European nations have been making mistake after mistake since WWII. Instead of rebuilding their own populations, they import “guest workers” who then DUH don’t leave and bring their families. They develop the EU and common currency that are highly dysfunctional and damaging to the less advanced economies. After the cold war, they let their militaries go to seed (except France, I guess). They embrace democratic socialism to the extent they emasculate their citizens will to do anything about it–maybe that is changing though.
I think there is a real possibility in my lifetime that one or more European nations will implode and no longer exist. My money on that happening is on Germany. They’re in real trouble. Sure it doesn’t seem like it now, but demographically it’s lights out in 30+ years. Like Hemingway said, the disaster will come slowly but then suddenly.