Typically, any change to a reservation after a category/points increase will require the new points value for the room being booked. If I were you, I'd call and verify before making the change.

Whether it changes and deducts additional points of cancels and creates a new one, the pointy impact is going to be the same.

The scenario of changing a reservation to a more expensive room while not charging for the difference isn't one I envision happening. Try it with cash - why would changing from a $100 room to a room that is now $150 be made without you paying an extra $50?