It's not only an issue with repricing and changes in point redemption levels -- that is a once-a-year kind of occurrence. But it's also worrying about free nights being unavailable later when you end up in effect cancelling and rebooking.

But the strategy you should take if you're not quite sure of end dates is to book the free nights as individual, consecutive, reservations, across the entire span. So you have all the room nights "in the bank" now, and you will cancel the extra nights at the beginning and/or end of your stay that you end up not needing.