We converted X (NF), which was the Business->First upgrade class code for the old UA, to ON which according to the linked chart is also the Business->First upgrade for all passengers.
The issue arises in that the old UA had only 1 upgrade class into First, whereas the new UA has 2, depending on how many cabins the flight has, so either way it won't be perfect. If you wish to delete and recreate the alert for a different class, you can do that as well. We will be sending an email to our users that have active UA Flight Alerts later today asking them to confirm the class codes on their alerts.