What train will you be taking and will you be booked into a sleeper?

If you are traveling in coach, business, or F on Acela. You should be able to call AGR and ask them to change the ticket for you assuming there is space on the other train. If AGR is closed at the time you may have luck getting an agent at the station to swap out the tickets for you.

If you have enough points you may want to consider making two bookings. You can cancel the one and have the points redeposited for no fee when you get to your destination.