I can't speak to the other carriers, but what has usually happened with me on United is that they refund the original ticket, since there's no penalty and then hit you for a new ticket.

If you insist, you can usually get them to do a ticket exchange in which case your receipt will usually show the full value of the fare, but in the detail fare calculation which is provided on the receipt, it will show a dollar amount, then A/C which stands for add collect. In a couple of cases the difference in fare paid has been more prominently displayed at the bottom of the receipt.