If you purchase directly from
DVC, they will only sell you points in your same Use Year. It doesn't matter when you decide to buy. If you want a different Use Year, purchasing 150 points or more would be considered a seperate contract and could be done. If a resort is approaching sell-out and there are no points left in your Use Year, DVC will not sell you less than 150 points in a different Use Year.
On the open resale market, you can purchase pretty much anything you want. However, it is expensive to buy small contracts and they are harder to come by.