Ok, let me see if I understand...
The trips are in March and May of 2003, right?
If the points you are using for the May trip are from your October '01 - '02 use year, then technically, they were banked points. You may not have specifically requested that they be banked, but if you had them available, MS will ususally use the older points first. (The determining factor is when the points are USED, not when the reservation is made.) If you cancel this reservation more than 31 days ahead of time, your points will be OK, but they must be used by September 30 of this year. They cannot be banked again.
If the March reservation was made with October '02 - '03 points, then they technically were not borrowed, for the same reason -- it's when the points are used, not when the reservation was made.
If you have to cancel your May reservation, I would recommend switching those points to your March reservation, since they must be used by September 30. You can always bank your current year points if you don't think you're going to use them.
If I misunderstood your use year info, disregard this and follow the advice of someone else.
