Strange. I've noticed for flights I've had booked for awhile with FFC that I can't change it past a certain point, but a few weeks ago I didn't have any issues taking a half dozen FFCs, booking a date before the expiration of the earliest expiring credit and then immediately changing it to something near the end of the current schedule. I have some more credits I'll need to do the same thing to eventually so will see what happens whenever I end up using them.