If you're boarding IN a bus station, you will probably get hit with the fee. They check the ticket (at least in NY or DC) before letting you through the door to the parking area to board. You have to have the correct ticket to pass which needs to be switched with the agent. I have heard no changes within 24 hours as well as $20 change fee but not the same answer 100% of the time. This is one of the reasons the lower cost carriers are killing them.