This is about all you can do. With credit you can extend by booking a new flight and cancelling. A new credit shell will get created. But, with vouchers issued by Frontier, they are gone on the date specified, no matter what. The ONLY way that changes is with the idea you proposed above. Try finding some flight that has very little weekly schedule, and book on a Tues/Wed far out. There's at least a probability of that going away if you book very far in the future.