I've been using WN since 1981, and the only time they've ever waived the fare upgrade rules has been when bad weather was present or expected. And yes....they've refused to move me from an oversold flight to an earlier "wide open one" on the same day.

My "favorite" incident was the time I got in twenty minutes early but they refused to let me walk across the hall to a half full connecting flight (I hadn't checked any luggage). No....unless I wanted to pony up a fare differential....I had to hang around for another 90 minutes to get on my originally booked (and completely full) connection.