You still need to make a booking for the flight you want. In addition to the possibility of not getting on the flight you want if you wait to long to book it, you also run the risk of the airline dropping the route. Without a confirmed flight/date/time you are unlikely to get the airline to reroute you for free.

Of course, you could ticket open and then make a bunch of bookings for what you think you will be flying. Bookings are easily changed.