What you do is book the cheaper itinerary online and pay for it in the usual way. Then call WS, cancel your old flights, and apply the credit to the flights you just booked. They will not charge you for the new flight (even though you paid for it online) and you will end up with a credit for the fare difference minus the $26.75 change fee ($25 + GST). Their telephone agents are very used to people doing this, so it is very quick and easy.

Unless things have changed, it is a $25 change fee per itinerary, not each way.