There is a "Book and Hold" option, but it's not always available, and it's only for paid tickets (not award tickets). I don't know if Premiers like yourself can use it. If it's available, it appears during the check-out process, after you've provided your Mileage Plus number and password. You can hold tickets until midnight of the following day, which is significantly less than the 72-hour hold available for award tickets.

Redepositing miles is $100. However, if you still want to use the award ticket, you have one year from the date the award ticket was issued (not the original flight dates) to reuse the ticket without a fee. You have to keep the same routings and airlines. If you redeposit the miles, you will be refunded the taxes.

Remember, if you decide to change the dates on your award ticket, be sure to do it ON OR BEFORE the date of the ticket's first scheduled segment. If you're a no-show, they'll cancel the ticket and keep the miles+taxes. (Same goes for paid tickets. If you don't call them the day of your first schedulaed segment, you'll forfeit the value of the ticket.)