If you have any kind of package from WDTC like Dream Maker, you will not be able to modify the UPHs at all.
If you purchase the UPHs at check in at your rersort, you may drop the departure day from your UPH.
There are plusses and minuses to both ways. If you get the UPHs as part of a package, you will pay the lower advance purchase price for the tickets for the entire length of stay. If you wait until you check in to buy them, you will pay the higher gate price but will have the flexibility to drop your first/last days.