So I got the same answer recently, regardless of what phone agents say you can definitely still standby for an earlier domestic flight, but only at the airport. Cannot be added to the list on the phone. Unfortunately I don't know about standby for a later flight though I suspect you can?

Online you can only do SDC, if your fare bucket isn't available you have to pay fare difference. I have had luck getting my fare class manually opened on a later flight by a helpful phone agent when it wasn't available online.

The other thing to note is the fare buckets change all the time, so checking 4 hours before a flight, the fare buckets may have opened again.