Don't stress! If they have been using it just.
Don't stress! If they have been using it in the past, and making a choice to change behavior, its all good! In a room that warm they won't need the brooder plate as much for sure. Also don't forget, chicks that grow up with brooder plates feather...
You might want to toy with the height of the plate. If its too low and they are too hot, they won't stay under it. Too high, they also won't stay under it. I set mine at an angle to try to help give them some choice of what feels best.