Sorry, but sadly it is not like buying a ticket to a sporting event. Once the ticket is issued in one person's name, they are stuck. As mentioned above, you will have to cancel that ticket and have another one re-issued for the new passenger. You would think since a body is sitting in that seat that it is no loss for the airline and they are double dipping since they will resell that seat, but this is the current state of affairs.
Good luck