If you really don't want to rebook, then you might to call. Certainly there are some things (like making a reservation with a free night certificate) which can only over the phone at Hilton (even though it can be done online in many other programs), so maybe a phone agent can do this even though you can't do it online. You can't tell unless you try.

I read this thread I did modify a points reservation online a few months ago, but in my case I wasn't changing the dates, the number of points required had fallen, and I was adjusting the reservation to get the new lower rate. And it did refund me the difference in points.