First up, ex post facto clauses have been interpreted as applying to criminal laws only. AFAIK there exists no constitutional bar to retroactive tax legislation. The same may be true for states.

And, It's not like they're all going to leave. Sure, a few grumpy ideologues like Musk will Go Galt and move away, but it's not like everyone subject to the tax is going to suddenly leave. People have been predicting the demise of California, New York, and so on forever, yet somehow, they have not all moved to Texas yet, and we still have plenty of high-earning taxpayers here. Maybe they like clean air and a functioning power grid, I don't know.