Tax attorney here. 1) there will be lawsuits about whether this tax can constitutionally be applied retroactively which the billionaires will be able to fund, so it makes sense to move before the proposition is passed so if that part is found unconstitutional, it doesn’t apply to you. 2) once a state comes after you, it’s rational to assume they will try to do so again, so moving makes sense both before and after the proposition is passed and if the proposition doesn’t pass and the proposition to ban future wealth taxes also doesn’t pass, it still makes sense to move. This really isn’t complicated and doesn’t take a tax LLM to discern. But you know, I do have one and I’ve worked in state tax law for over a decade. I do advise people (but more often corporations) on state taxes and this is exactly what I would explain to a billionaire if they were my client. I would also advise them that they would have a legitimate lawsuit about whether taxing unrealized gains is constitutional at all but that probably goes in favor of not moving if it impacts their decision to move at all.