First, are you sure the change fee is per direction? Ive never seen an international flight with per direction fees except for N/A, central America.

Second, there's no such thing as a credit voucher for non refundable tickets. I think the agents just keep the old tickets and exchange them. Quite a big issue when changing a euro ticket to a domestic flight. Agent said something about YQ not being applicable to base fare on ticket exchanges.

As for the 400 cancel and 600 changes, it's for people who have already left and have no choice to change the return. Or someone that bought a P fare that is no longer available and just want to change their return (fare rules say that changes to the return is done using historical prices.