I would be very careful about throwing away a coupon and not first going to the airline to change your booking. Airlines have a habit of cancelling entire bookings paper or not. We live in a much more connected world now.

Also remember that most OW alliance members (minus CX and AA) use Amadeus so they know what has and has not been flowen regardless of whether you have a paper ticket rather than e-ticket.

I would be very careful!

Perhaps I am being overally cautious, but I still think it is better to be safe than sorry. If you want to change your ticket, go and speak to the airline first.