Both the cash and points portion of your payment for your stay will reprice if you modify the stay. So to answer your question, it is dependent on the current pricing versus the pricing at the time you made your original booking. To see the current pricing, start a dummy reservation for the nights you intend to stay and compare to your original pricing. If it is favorable, go ahead and make the change. If not, consider the fifth night a guaranteed extra late checkout. Of course you can continue to check for favorable price changes anytime before your stay. I do this for most of my reservations and have saved both a lot of points and cash by doing so. Good Luck!

--Jon

P.S. While you could try just directly modifying the original reservation to check current pricing, I prefer to do it with a dummy reservation. Less chance for system errors. If I have enough points available, I will complete the test reservation and then cancel the old reservation. If not, then I modify the original reservation once I have confirmed better pricing is available.