All Northeast Regional tickets are "reserved." Thus, if you want to take an earlier train, you will need to change the reservation. I think you can do that on the app. Otherwise, there is an understaffed ticket office at Washington Union Station. The fare for the changed reservation may well be higher than your original, but now always. When I take my day trips on the NEC, I tend to keep my return open and don't book until right before travel. In the last couple of years, I've been pleasantly surprised at what I can buy at the last minute. Note that on the Northeast Regionals, coach is open seating, but in business class you're assigned a seat (which can be changed by the traveler.) Also, be aware that lately, I've been seeing many Acela business class fares that are cheaper than the Northeast Regional business class fares. I think that ridership on the Acela hasn't been recovering as quickly as ridership on the Northeast Regionals.
