You don't even need to actually get a supervisor - just ask for one. Almost always, another agent (one that knows what s/he is doing) will get on the line and fix it in a few seconds.As Ryan said, call back. If the agent says s/he can't do it, ask for a supervisor. The supervisor should know how to MODIFY your ticket (don't let them cancel/rebook it). By modifying it you'll keep your bedroom.
Keep trying. Explain to them that you are already booked to occupy one of those rooms that are "sold out". You just want to MODIFY where you board - one station later. Again try asking for a supervisor.2 Amtrak agents told me because the train was sold out my boarding station could not be modified which seems crazy to me.
ABQ is my home station. A ticket from someone boarding in ABQ will be scanned before the train gets to Lamy for sure. It is 79 miles to Lamy and driving a car on the Interstate (speed limit 75 mph) takes a little over an hour.How close is Lamy to Albuquerque? Would he risk being a no-show before he boards and has his ticket scanned?