Amtrak tickets are generally fully changable.

You can buy a ticket for one time and use it on another.

If you change it at the ticket desk, you will have to pay the difference.

But if you simply go to the train, you can show it to the conductor as you are getting on. The conductor will 80% of the time wave you on through, so long as your earlier ticket is for the Unreserved train and not for Acela Express or Metroliner.

In other words, you can't use your unreserved ticket on a reserved train (Metroliner or Acela Express), but you can probably use it on another unreserved train, even if the time is different.

The conductors have a lot of discretion here.

Don't try this without showing it to the conductor as you get on.

BTW, just to be very clear, your ticket is probably for the Acela Regional unreserved train (although it may not say that).

The Acela Express is an all-reserved train with a confusingly similar name and more than 2X as pricey but not all that much faster.