Looked at the web site and the bottom line is as of January 3, instead of departing Seattle at 9:45 am, the Coast Starlight departs at 11:45 am. That's nice, I won't have to rush to leave the hotel.
Not much of a change other than that. Arrival at Salem, OR may be after sunset for this trip, which means no photos of the shiny statue on top of the capitol. It'll be dark at Eugene and through the rest of Oregon and NorCal, but it ought to be light at Chico, which it would not have been under the previous schedule.
For those going to LA, it makes the arrival 11pm instead of 9 pm, which could be an issue, though not for me. :-)
The new schedule is not on the web site yet (the latest one was just published in November), so I don't know if they have done the same for the northbound route.
I'm guessing the change was made mostly to help the connection from Vancouver BC. Yup, as of 1/3 the Amtrak Cascades train starts running from Van to Seattle, and arrives at 11 am.