I wonder if it depends on the actual ticket agent.

I was one day scheduled on a late afternoon acela out of PVD. I got to the station, and then an announcement was made that the train was going to be late. Half an hour later, I was able to confirm that the train still had not made it past BBY.

I just went up to the ticket counter and the agent happily switched me to the next acela. I don't know if it matters that I was ticketed for FC?

Interestingly enough, however, eventually they flat out cancelled the train and just allowed everyone with a ticket for my original train to board the next acela, no re-ticketing needed.

let's just say that we were above capacity in FC and that someone was sitting in that weird little phone booth.