I just did this, here's what MS told me: If you booked your reservation less than 7 months out, they can just reallocate the points without rebooking. If you booked more than 7 months out, they have to cancel and rebook.
We were in this situation on one of our add-ons and they dealt with it right while we were in the Sales Centre. Another time it dealt with developer points and banking...can"t remember the specifics as it was very confusing...the change was made by staff other than MS. they didn"t have the authority. It took a while to clear but it was eventually done.
We were able to switch points after booking our trip and then doing an add on for 100 points. All points were for the same resort and we switched our reservation to include the add on points. We had originally booked the trip in Jan and switched points in April, which was eight months out.