Whenever changing the dates or the number of nights for a stay, the rate can change. The new rate will depend on what the revenue management software is predicting for occupancy during your stay when you try to make the change.

For your situation, I recommend monitoring the rates up to check-in time. Sometimes rates go down again as it gets closer to a stay depending on the projected occupancy level. If they do, just re-book via the web (modify the existing reservation or do a new one and cancel the old one). Finally, if that does't work, you can politely ask the front desk associate at the time of check in. They have a lot of discretion and may be willing to adjust the number of nights for the stay at the same rate but are under no obligation to do so.

--Jon