Of course, in most of the world you don't need a reservation, standees are allowed, and passengers operate the automatically-locked train doors at the stations, too. Conductors (or the local national term) routinely sell tickets onboard, except on certain routes where a substantial penalty will be incurred.

A few years ago I had a Business Class Acela ticket and arrived at Boston South Station really early. I was able to move into Business Class on an earlier Northeast Regional for a mere 40 cents. I remember being taken aback when the agent asked me for $.40 for fare difference but I laughed, handed him a quarter, dime and nickel and left early.

Update
 

Just want to report my experience. I was able to get the Amtrak ticket agent at EWR to issue a ticket for the earlier train. Not sure if I got lucky or that was within the rules.

LAX

There is nothing against changing reservation to an earlier train. You just use the credit from the existing ticket, and pay against the fare of the new reservation. Since it's last minute, it's probably higher. In your case, it definitely was. A regional business class ticket is typically quite a bit cheaper than a business class Acela ticket. Also, you'll probably earn fewer AGR points with the regional.

You pay a fare difference, if there is one. Or, in the rare case, you get a future travel credit voucher.

Thanks for the report. I presume it was all within the rules, as the CO codeshare ticket likely books into a fare basis that is fully changeable, that is, exempt from yield management. Do you happen to remember what fare basis was on the Amtrak ticket? It's on the ticket stub handed back to you by the conductor. (Typically something like YE, YD, YB, YA, or Y for NE Regional coach.)

It was YA. I have no clue what that means, though!:D

LAX

In a lot of Europe people are travelling on unreserved flexible tickets, though. That said, not everyone is, and a lot of high speed services e.g. TGV require reservations.

Neil

There are 5 bucket or price levels for that train, and most reserved coach trains for that matter. Y is the most expensive price, followed in decending order by YA, YB, YD, and YG being the lowest price.

Hey, it was CO that paid Amtrak. My plane ticket would have costed me more flying into PHL instead of taking the train to ZFV, which was what I needed to do from PHL anyway (on a commuter train instead, though)!

LAX

Yup, as AlanB reported, this is the 2nd-highest coach fare class. (Call it the equivalent of a "B" fare on most airlines.)

Assuming that CO booked you into Y or YA on your original train, it's straightforward and common to rebook into YA on an earlier train. No change fees on Amtrak. I could have sworn that I once received a YD ticket through CO for PHL-EWR. Back then, unreserved trains still existed, so I used my reserved YD ticket on an earlier, unreserved train. No harm, no foul.

For the curious, the folks on the CO board may have more insight into how CO books and pays for Amtrak tickets.