I believe that you're talking about two different things and trying to equate them. What HHonors is referring to in your quote is an early checkout penalty, which some hotels will charge if you choose to end your stay after it is already started.

This is different than a hotel changing rates based on lengthening or shortening the length of the reservation before the stay commences. That said, this happens at Hilton from time to time, and it generally reflects different price availability at the time of the change as opposed to the time you made the reservation. (Your new reservation may be more or less than the original one, depending upon the going rate and room availability at the time of the change, or it may still be the same.)

Unless your rate specifically states otherwise, there is no penalty - such as what you are describing - if you wish to change or cancel your reservation before the fact. You may end up paying more (or less) for the room, but that is dependent solely on availability and price at the time of the change.

Mike