You will need to cancel the whole thing and have to rebook. The risk you run is if the standard rooms go back to availability immediately. You also run the risk of the points reposting immediately. Although others have reported no issues, I have had consistently had my points NOT go back to my account immediately. And one time I needed the diamond phone rep call the property to have them manually put the standard room back in the inventory.
Things to keep in mind. Ultimately, it comes down to how much time you are willing to devote to being on the phone dealing with this and how much you are willing to risk losing the standard room nights/5th night free.