No, you can not move back to your 2nd round seat even if you have chosen the float option. The float option means that you will accept the seat you were given in 2nd round but will change if a better seat is offered in the subsequent rounds. If you get a more preferred seat in the 7th round of counseling, then you can not go back to your previous seat. If you are satisfied with your second round seat, then it is better to freeze the seat.