This app lets organisers of KoorTickets-events swap the products of two eligible tickets. Over-the-counter sales can then be handled more easily by making use of pre-printed "PlaceHolder" QR codes and swapping the contents of these with QR codes just generated by the KoorTickets ticketshop. This obviates the need to print out the just-generated ticket: a pre-printed ticket can be "filled" with the content/products of the recently sold ticket.