They have guaranteed week offerings. If you choose you do not want to use that week at your home resort, you can then use the points at a different resort or a different time. The guaranteed week is only for the home resort you purchase at. If the point cost for that week goes up in the future, you are exempt from it. If it goes down, I do not know if you have a few point surplus in your account at that point. That is a question for DVC. Guaranteed week is only secured via resale of a guaranteed week or a new point contract sale direct from Disney. They do not commonly pop up on resale, however.
Because you are buying the convenience of guaranteed, typically it is in a higher demand time of year, you pay an additional premium on top of what ever the point cost for the seven nights is. That being said, all points for the guaranteed week are on one contract and cannot be broken up in to smaller contracts.