A revision to the SDC policy has gone into effect. Previously, you could SDC within 24 hours of departure of your original flight, provided the new flight was on the same day as your original flight.

The policy has been amended to only allow you SDC to an EARLIER flight than your original one.

same rules requiring available fare class for purchased Main and C+ // last seat availability for FC and Domestic D1 remain.

this is a major devaluation of the policy and severely restricts abilities to make changes. Essentially, if you are booked on the first flight out, you will never have SDC available to you.