AGR will probably be closed on Monday for the holiday (since the call center is in Canada now, maybe they won't be closed, but in the past they've always been closed on Memorial Day). A full refund shouldn't be a problem, though: AGR will be able to see the ticket was never printed.

You have two options if you want to travel before Tuesday when AGR reopens:
a) book a new AGR ticket online and then get a refund of your existing reservation later. This, of course, requires that you have sufficient points in your account.
b) attempt to get an Amtrak agent to change your existing reservation. When AGR is closed, they are allowed to do so, though not all agents know that, nor do they know how to do so. It is, however, possible.