If you are just trying to reduce dues etc., it might cost more than it is worth. Selling a 420 point contract and purchasing a smaller point contract would end up being costly.
What is your purpose? Are you looking to reduce costs, and points, or are you looking for a way to buy into the new longer contract? I have heard of some doing that too. If your 420 points are in more than one contract, then you just sell one contract, but if all your points were purchased at the same time on the same contract, it must go as a whole entitiy.