After making a reservation, you can drop any days at the beginning or any at the end (or both) while retaining the same reservation number and avoiding any risk of losing the reservation. You will need to either call or email Member Services to make that change because the online booking system is not set up to allow any modifications of a reservation once it is made -- the only thing you could do online is cancel the whole reservation and then try to rebook a new one which could risk ending up with no reservation if someone else gets what you want during the time in between your cancelling and booking again.
There is no fee or prescribed penalty for making the change but if made at a time beyond the eigth month of your use year, you would be blocked from banking any current use year points taken out of the reservation, and if the change is made 30 days or less from date of arrival, all points taken out of the reservation would go into a holding account and they could only be used for a new reservation made 60 days or less from arrival date and they cannot be banked.