You always have the option of purchasing additional points at any time when you need them without having to re-write your entire contract.  When you signed up for the service, the contract provided you with an overage point rate.  If you go over in points and either don't have Rollver & Borrow, or you've exhausted Rollover & Borrow that month, then the overage rate will automatically apply to the points that fall into the overage category.  Overage charges only apply when you actually go over.  If you don't go over in the month that follows, there is no overage rate.