This is a reasonable concern. The most thorough analysis I've seen (also mentioned below) projects that increased tax avoidance will have a larger impact than people moving out of state. But taken together they won't be quite enough to make the impact of this change negative:

"Together, cross-border moves and tax avoidance would reduce millionaires tax revenue by roughly 35 percent [to $1.3 billion]. (Absent these responses, the tax would be expected to raise $2.1 billion in 2023.)"

Via: https://cspa.tufts.edu/node/406