I don't know whether this is common knowledge. I just "changed" my ticket to the exact same ticket on a long-distance sleeper, and got a $350 refund in the form of an Amtrak voucher, due to prices near Christmas having decreased from when I bought the ticket.

This seems to be a combination of three policies:

1. Amtrak has a temporary "free changes" policy for covid, with the caveat that you'll be responsible for any difference in fare

2. Surprisingly to me, if the new fare is lower, they will actually issue you a voucher for the difference, not only upcharge you for increased fares

3. Also surprisingly, you can "change" to the same exact ticket as the original one. I did this for a long-distance sleeper reservation, and it even kept me in the same room. This avoids needing to play 2-step games like changing to a new date then changing back to the original one. Just go through the change-ticket process, and select the same itinerary you already booked.

The upshot is: If you booked a ticket whose price has dropped, you can "change ticket" and get the fare difference back (as an Amtrak voucher).