This is where the Use Year comes into play.
You will probably get more detailed info from more experienced
DVC'rs, but the ressie would fall into the 60 day rule, where you can only use the points to make a ressie within 60 days of using the ressie. Also, you would have to use those points b/4 the end of your use year, so if the ressie is for December, 2006 and your UY is February, then you would have to use the points b/4 1/31/07...not a good situation.
There is more to it than this, as it depends upon whether the points were banked or borrowed, but suffice to say that cancelling a ressie within 30 days of arrival is not a good idea and cancelling a ressie just b/4 the end of your UY is also not a good idea.
I have also heard that Disney does not want to be known as the Grinch who stole christmas, so depending upon the reason why you are cancelling the ressie you maybe able to work out a deal with DVC if you are put into a situation where the points are about to lapse.