Excellent that you have not yet printed the ticket. I am in this position on a regular basis and do the following: if it looks like you are going to miss your train, even if it's ten (or two) minutes before the train leaves, call 800-USA-RAIL on your cell phone, say "AGENT," and change your ticket to the next train. Yes, you will most likely lose your AAA discount, but you will get a new booking right away and not have to bother with standing in line at the ticket window. You just pick up the new ticket at the Quik-Trak machine, which will "know" about the change instantly.

For this reason I also ALWAYS buy my tickets as one-ways, not RTs. If you need to change one leg of the trip, it is far easier to do it via phone/Quik-Trak than by standing at the window, which would be necessary if you had paper tickets in hand.