Thanks Zink,
I was thinking that I was going to have to cancel a stay, due to a conflict, and then rebook using new dates/new redemption schedule. Now I'll try to make a change under the old cert. I can guess that while maybe Hilton doesn't want to get adversarial, aka petty, over changes, on the other hand they likely didn't want publish that certs can be changed and then have to deal with the fallout of folks making a bunch of semi-random reservations and then massaging them as needed.