Be careful about this. I had a situation where I asked a Plat Concierge very specifically and she said she was sure that she could change the reservation without cancelling it. I verified this several times before telling her to do it. She made the change and, sure enough, she cancelled the old reservation and made a new one, which was not what I wanted.

I think the OP wants to change to a better room type but still pay the old (lower) number of points. I don't think this will work because it will be a new reservation and thus require the new number of points.

IMO the best chance would be to contact the hotel and ask (this would be a favor) to get the better room type for the same number of points (state it clearly, including a clear mention of the exact number) as the current confirmation shows.