I keep reading that NYers are flocking to FL and TX, states they usually hate, due to Covid. Are these people going to stay there once big law and the financial sector require workers to return to their offices? Will these people try to sell their houses in FL and TX then or keep them as vacation homes?

Why do you assume they are all in big law and financial? That is very narrow minded of you. Also, housing is much less expensive in Fla and Texas which is part of teh reason people are leaving NY.
People I know moving to those places are either moving to satellite offices, have family, or have lost their jobs.
These are people who already wanted to do this before covid. That's the only explanation.
friends of mine moved just for the year for schools. Even with much higher cases they were able to have kids in school.
Why would you move to a covid hotspot "because of covid"?
N.C. Is inundated with NY’ers and people from CA. We just bought and made a permanent move from the city.
God, just about nothing sounds worse than living in either of those places. Have fun, y'all.