No.

Whether a reservation is modified on the website, on the app, through central reservations, or by someone at the property, the modified reservation will be based on current availability and current pricing (including points pricing),

This includes changing the length of the stay, the check-in date, the room type, and even the number of guests. In fact, it's not even possible to replace a points night with a certificate night, even if not changing anything else.

Sorry to be the bearer of bad news.