This is a great perk which we have taken advantage of many times. We sometimes book 330 days for high season or very hard to get and then keep trying to perfect as we get closer to the date. We do a lot of awards that are more than one flight so it is also good if one piece isn't available at time of booking it can be added or subtracted at a later point in time. We are going to Africa next month and after months of looking for jnb to Victoria falls availability it opened up last week and we had the awards rebooked. Since it was an end point there were no extra miles and a very nominal tax.