You are correct. So, do what I have done after moving out of NY three years ago: Keep your NY 529s in NY, otherwise I'd owe New York State just shy of $5,500 in NYS taxes that I never paid because of my $10,000/year state income tax deductions. They will know when you move it out of the NY account because they have oversight of the NY 529 plan and they know you're no longer filing taxes as a state resident.
One good aspect is that you can make take the NY 529 tax deduction in full (up to $10,000 joint filier) even for the year you move. Say you contribute to your NY 529 in March and move out of state in July. You were a resident at the time of the contribution, so you get the full deduction. If your new state also grants tax benefits to their 529s (and if you like the plan), you can also benefit that year from the new state's deductions as well--if you have the $ to do it and you do it now as a new, full-time resident of that second state.
Anyway, in my mind, there is nothing wrong with a little extra diversification by having money in two different state plans, and with keeping the previous NY tax-deduction benefits. You will NOT owe back any taxes to NY once you make NY 529 qualified withdrawals while a resident in another state.