Disney policy has been that add-ons must be in the same use year as your main contract. If you wanted a December use year you would have to buy a new 150 pt minimum contract. The good part of that is that 150 pts makes you eligible for the extra 100 pts that they are giving as an incentive.
The 100 point incentive for purchasing 150 points doesn't sound like the best deal to me. The $10 credit would come out to $1500. You could then go rent those 100 points for $1000 and be $500 ahead.