You can check with the airline but I have been in a similar situation. An AC airline agent told me to phone to say I can't make that flight and reconfirm me on the other segment that I will be going on. It worked and saved me a Saturday night stay and about $1500 once (too convoluted to explain here).

The airline will cancel your itinerary if you don't show up for the first part of a series of flights unless you cancel the segment in question. You will have to ask your airline what their policy is as on some, it is just not allowed under their rules (i.e throwaway coupon). Non North American airlines may not have such stupid rules.