Right now Sunset NOL departures through Saturday, 9/4 are cancelled. Monday, 9/6 and Wednesday 9/8 are still showing as running. This could change. For example, if New Orleans remains without power they will be unable to service the Los Angeles based train and they will cancel more runs east of San Antonio.
You will have at least a couple days notice. The consist departing Los Angeles on Sunday 9/5 will be turned and become your train. They will likely decide before it departs LA on 9/5 whether they will be truncating that run at San Antonio.
If your train is cancelled, you will get a full refund. If you wish to cancel now, call and talk to an agent, they may be able to work something out. If you try to cancel on the website with a train showing as operating, you will definitely be charged the penalty.