If you're departing from a staffed station then the easiest thing to do is ask the ticket agent if you can switch the ticket. I think that convincing the conductor really depends on the conductor. If the train is full then I would not recommend boarding with a ticket for a later train. If it's an empty train then asking the conductor as you board is probably your best bet. Of course if you missed your train and are taking a later train, then you have a much better shot at being let on the train by the conductor. If you have a business or first class seat then I would never attempt to get on a earlier train as those seats are limited and have a greater probability of being sold out.