I have a tight connection and may miss my train so are the self-service machine easy to use for changing a reservation. I plan on buying on the web ticketless so I will have a PNR and CP codes to make the change.
Lets be sure we know what we are talking about. If you buy a web ticket at discount price that ticket is only going to be good for a specific train and time. Miss that train and ticket is no good. If full fare, non-discounted ticket, you have about 90 minutes to see a ticket agent and get the ticket exchanged. Cannot do that at a ticket machine.