I think this is a scam that Frontier pulls, dangling a $1000 voucher in front of people in order to convince them to give up their seat but not telling them the voucher has to be used all in one transaction. Not many people can book one reservation and use up $1000 and they know that.
This is good information to know. If this opportunity is ever offered to me, I will ask if they'll issue it as four $250 vouchers. If they no, I'm not giving up my seat.