Actually, it is not 15%. It is the annual dues that cannot change more than 15% a year, absent an actual vote of the members.
As to points needed for a room, the annual change cannot exceed 20%. up or down, for any given date, except that there is no limit to total points per night, up or down, that can be made for key holiday periods that change dates annually -- the two Easter weeks (Sat a week before Easter to Fri after Easter), the three Thanksgiving days, or at some non-WDW resorts, the beginning or ending dates of the Christmas and some other seasons In any event, no matter what changes are made, total points for the year need to remain the same.
Be aware that other than the usual change you get by part of those holidays changing dates, there was a major change at WDW exceeding 20% made for the Monday to Friday of Easter week (but not week leading up to Easter or Easter ) beginning 2021 -- that Monday to Friday has now moved into the season that is below Premier Season.