Just did this recently and changed a May reservation by dropping last night and adding a first night. There is no particular function to just do that. The modifcation system for dates works the same way for all changes. You go to the modify page for your reservation and choose to modify the reservation. You then create a new reservation for the same resort, room category, and the new dates you want (the entire time you will be there). You then confirm that change (if available), and you will have the new reservation (same number as before) and the old dates are no longer.
Checked availability before making the change and four of the nights I wanted to keep were shown as unavailable and new night I wanted was shown as available. So though one is seemingly changing the whole reservation, the nights you already have and want to keep do not have to be separately available.