In the situation you mention, UA could reprice existing fares or open special new lower fare class/basis with the desired fare.
Note an S fare 3 months out will be priced different than a S fare 2 weeks out and different from a S a few days before travel. Each of those will be a different fare basis.

Note the opening of a fare class but not having a valid fare basis means no new tickets can be sold. But rather it makes changes / SDC possible.