I thought I'd come back and let you know what I ended up doing.

I left early and ended up cancelling the return (which was booked separately from the outbound) and was going to see if I could make the train before it, but it was leaving while I cancelled the ticket, so I took the commuter snail instead. A friend of mine happened to be on the earlier Amtrak train, and told me about how he got home an hour before I did!

Well, luckily, I wasn't in a hurry! The ticket was cancelled and a full value eVoucher was credited to my account. I ride Amtrak often enough that I will end up using it sooner or later.