If they allowed this, wouldn’t every guest with a non-prepaid booking be able to try and reprice their reservations day of arrival, if rates had dropped?
every guest should. I certainly do. I can't recall a single time a Marriott or Hyatt property has declined to change my (non-prepaid) reservation to a newer, lower, rate (assuming identical room type, terms etc). Not that it happens that often, but certain properties (e.g., Courtyard Boston Logan) can drop rates by $100+ within a day of arrival. For Marriott, I call the property directly; for Hyatt, I call the glob line. This is definitely not the kind of thing I would reasonably expect to be as successful in the app chat. I save that for "I'm arriving after midnight," "do I need to call for the shuttle when I land," "I would appreciate an upgrade to x," etc.