Well Amtrak has the room number listed on the ticket (which I don't have any problem with), so how could they not send you a new ticket if that changes? They don't change the reservation number or anything like that, so I just don't see the problem with that system. I do have a big problem with the fact that so many Amtrak agents have such difficulty performing reasonably basic tasks, but the system itself seems perfectly fair.
Given that all Amtrak tickets are e-(for electronic) tickets, you never see a ticket these days. What you're seeing is printed evidence of an e-ticket but not the actual ticket itself. But in the days of paper tickets, tickets had lots of things on them that were not part of the legal ticket.
But to understand what I mean by "legal ticket", you need to understand that historically (and still today for airlines), a ticket was a negotiable instrument much like financial negotiable instruments such as a check. Any time one carrier issues a ticket for use on another carrier, there has to be a way for the transporting carrier to be paid by the issuing carrier. And that way was for the transporting carrier to submit the ticket to the issuing carrier for payment just like how banks clear checks. For instance, back in the day, let's say you took the Broadway Limited and then the Super Chief from New York to Los Angeles. You could purchase your ticket for the entire trip from the PRR in New York. PRR held all the money but then when you were on the Super Chief, the ATSF conductor collected the ticket for transportation on his train. He turned it in at the end of the run and eventually the ticket made it to Santa Fe's settlement office which submitted it to PRR for payment. Collecting and protecting tickets was as important as collecting and protecting cash as foreign tickets were a cash equivalent to the railroad. Even a single train that operated on multiple railroads (such as a NY to Florida train or any Chicago to West Coast train except on the Santa Fe would have separate tickets for each railroad (that was one of the reasons behind SF's "Santa Fe All the Way" marketing slogan).
When you travel, you need a ticket and if the train or plane requires reserved space, a reservation. They are separate things but since tickets frequently reference reservation information, people get confused. A ticket can be unrestricted (good on any train or plane until the ticket's expiration date) or have restrictions such as not valid before/after, not valid on certain days of the week, or be restricted to a particular train/flight and date. The latter is usually true of any sort of discounted (anything lower than the top price bucket) ticket. As I said above, the ticket might have additional information on it that is not officially part of the ticket. For instance, let's go back 30 years and say I make a reservation and buy a ticket to fly from NYC to Chicago. Since I know my plans might change, I buy an unrestricted ticket (good on any flight on any airline). None the less, the printed ticket most likely showed the reservation information that existed at the time of ticketing. But at the airport, that meant nothing since the reservation might have been changed in the interim. Checking in for the flight, they'd check to see if I still had a reservation. If I needed to make a change, they'd change the reservation but there was no need to do anything to the ticket because even though it had "for information only" reservation data on it, the ticket itself was good on any flight on any airline between those two cities until its expiration date.
Which brings us to Amtrak today. Above, I said a ticket can be restricted to a particular date and train. But based on the empirical evidence, Amtrak restricts tickets to a particular date, train and space. Hence the hoops to be jumped through to change space and too many Amtrak CSRs saying the new room is more expensive since making the change violated the ticket restriction and hence a ticket reissue or revalidation is required. I may be wrong but I really can't think of any other good reason why CSRs would claim a different room costs more unless the ticket is restricted to the specific space being held.
One last thing regarding reservation numbers (officially known as PNRs in the industry for Passenger Name Record). It's best they don't change when you make a change since that then keeps the history all in one place. A PNR documents a reservation and should have a reference to the ticket being used to travel on that reservation (but not required - back in the day, at least on an airline, a passenger holding an open unrestricted ticket could make a reservation and leave it unticketed and then present that paper unrestricted ticket he was holding at check-in) but it is not hard-tied to a particular ticket, pair of cities, or even passenger - it's just a record with a history of every change made to it.
