That's what makes me suspicious, it is seemingly on a lot of flights until then. I haven't tried to book yet though, since I already have other flights then that I'd have to cancel and refund for the miles first.
You can just change your existing reservation. Click on change reservation (or click the ellipses to the right of the reservation in the APP). From there, choose cancel trip and use the value toward a new trip.
Enter your new itinerary. The results will show the difference in miles and fees; either could be an add/collect or a refund. This preserves the $12.50 booking fee.
James