I called Amtrak, and the reason I couldn't do anything with the remaining leg of the trip was because there was a 17 minute schedule change. That is silly - why should that block any changes or even seeing the reservation on the mobile app? In any case, they accepted the change on their end, and when they went to cancel it offered an eVoucher significantly bigger than the voucher amount used to pay for it ($83 instead of $47), and significantly lower cash refund than the original cash amount paid ($13 instead of $49). While I can now see/modify the reservation online, it shows the same cash/voucher breakdown when cancelling on the website. As the regular Amtrak agent couldn't override this, I was transferred to Customer Relations, though they closed before I could get through. Guess I'll have to call back tomorrow - may see if they can just refund the vouchers, as those funds were from repricing my since-cancelled trip that fell under the medical cancellation policy.