I did this the other day. I just got on and showed the ticket to the conductor who said I was fine.

The fare on the unreserved train between PHL and NYP is the same whether you are on the Keystone or the Acela Regional.

The difference is the times, which is printed on the ticket, but the conductors usually don't split hairs over that. They are mostly concerned about unreserved tickets being used on reserved trains.

You should be fine with the ticket as is, on the later unreserved train.